Chapter Text
It still didn’t feel real.
Katsuki stood at the edge of the observation platform; eyes locked on the slow sway of seawater beyond the reinforced glass. The lighting was dim, filtered like sunlight through deep ocean layers, and it cast the entire chamber in a cold, greenish hue. He hadn’t seen one yet, not really. Just the hint of movement, a flick of a fin vanishing behind artificial reef, shadows long and fluid. But they were here.
And so was he.
Yuuei Aquatic Rehabilitation and Research Facility wasn’t just any marine lab. It was the place—unreachable for most biologists, impossible to get into without years of fieldwork, spotless recommendations, and a ruthless interview process that tested more than knowledge. You had to fit, they said. You had to understand. Whatever that meant.
He’d gotten the call two weeks ago. Still didn’t know why they picked him. Katsuki wasn’t exactly a people person, and if the rumors were true, merfolk were even less tolerant of bullshit than he was.
He adjusted his badge—Dr. Katsuki Bakugou, Habitat and Dietary Specialist—and turned away from the tank, focusing instead on the clipboard in his hand. Meeting with supervisor in Enclosure 3B, the largest of the Quarantine tanks, currently housing one “recovered” merfolk.
Recovered. That was Yuuei’s word for it. Not captured. Not rescued. Not relocated.
Recovered.
He wasn't sure if it was supposed to sound gentle or clinical. Probably both.
"You're early," a voice said behind him.
Katsuki didn’t turn. “I’m thorough.”
He heard the soft thud of boots against the walkway as someone stepped closer. “We like that here. But let me give you some advice—thorough doesn’t mean loud. Especially not with them.”
Katsuki finally glanced sideways. A man in his late thirties, sharp-eyed, dressed in a plain uniform with no visible rank or badge. Staff, clearly. Probably someone higher up.
“They’re sensitive to sound?” Katsuki asked, already jotting a note.
“They’re sensitive to intent,” the man said. “Name’s Aizawa. I’m an on-site caretaker. You’ll be working under my clearance for now. Don’t wander. Don’t tap the glass. And if you see them watching you, don’t stare back.”
Katsuki raised a brow. “Why not?”
“Because you’re not the only one studying them.”
That gave him pause.
Aizawa nodded toward the tank behind him. “They’re more intelligent than we understand. Some talk. Some don’t. All of them watch. You want to learn about their diet and habitats? Good. Just remember—this isn’t a zoo. It’s their space. You’re the visitor.”
Katsuki nodded once, serious. “Got it.”
He was used to dangerous environments. Harsh conditions. Predators. But this—this was something new. Something ancient. Yuuei wasn’t just a lab, and the merfolk weren’t just specimens.
The facility was even bigger than Katsuki had expected.
Aizawa didn’t waste time with small talk as he led him through the main corridor. Thick glass walls framed the interior, all looking out into various tanks and simulated environments. The air inside smelled like salt and antiseptic, sharp and cold. The lighting was low in most areas, mimicking deep-sea twilight, though here and there, sunlamps cast gentle beams across certain habitats—controlled sunlight for controlled growth.
Katsuki walked slightly behind, eyes flicking from panel to panel as Aizawa spoke.
“This wing houses the research labs. Bio-sample processing, dietary testing, sonar behavior analysis, and environmental control for the tanks. You’ll be using lab three—diet and digestion.”
Katsuki glanced through the glass window into Lab 3 as they passed. It looked sterile, spotless—equipment gleamed beneath sealed lights. Dozens of screens and tanks lined the walls. He spotted a refrigeration unit labeled with colorful warning tape and some notes in messy handwriting. Someone else's research already underway. He'd be stepping into a rotation, not starting fresh. That suited him just fine.
Aizawa kept walking. “Observation rooms are along the upper deck. Two-way glass. You’ll have access to C-Level permissions for now, but that may change depending on how things go.”
They passed a pair of techs speaking quietly into comms near an access hatch. One glanced at Katsuki with a raised brow, then looked away. That was fine. Katsuki didn’t care about the stares—he knew what they were thinking. It was nearly impossible to get hired here. Especially straight into a specialist role. People worked their whole lives to get in the door at Yuuei. He’d been given a shot—and he wasn’t about to waste it worrying about egos.
He followed Aizawa into a wider hallway that opened into what looked like a large interior chamber. It took him a second to realize the wall across from him wasn’t a wall at all.
It was water—a massive indoor tank that stretched from the ground floor up through multiple levels. The bottom was dim, studded with coral and stone, and the top shimmered in the overhead light.
“This is the social pool,” Aizawa said. “We don’t keep the Mer in individual enclosures unless medically necessary. They aren’t animals. Most of them prefer this space.”
Katsuki stepped closer. The water wasn’t crystal clear—it held the faint green-blue haze of natural ocean water. This wasn’t some sanitized aquarium. This was a functioning ecosystem. Sea grass waved along the lower ledges, and bioluminescent strands curled against fake reef rock.
“It’s connected to the reef?” Katsuki asked.
Aizawa nodded. “Outdoor passage leads directly to the cove. It's unmarked. Camouflaged. They can come and go, but most of the ones under care stay close.”
Katsuki eyed the far wall where the water darkened. Somewhere beyond that glass, the tank opened to the wild sea. No gates. No barriers. Just the open ocean.
A massive stingray glided past below the surface, wings curling. No—wait. Not a stingray. That was—
“Spotted one yet?” Aizawa asked without looking.
Katsuki nodded once, watching the shape disappear beneath a rocky overhang.
“Don’t expect them to approach,” Aizawa warned. “Especially not the adults. The younger ones are more curious, but they’re still wary of staff. They’re not used to new faces. Especially not humans poking around their diets.”
“Tch. I won’t poke. I observe.”
Aizawa gave him the faintest look—approving, maybe. Or just amused.
“Good.”
They moved on. Past more observation decks, a couple of training pools, and what looked like a nursery behind closed glass—he didn’t ask questions about that one. Aizawa didn’t offer any.
Eventually, they reached a reinforced stairwell that spiraled upward.
“Top deck is staff only,” Aizawa said as they climbed. “Surveillance, data logging, backup generators, and weather monitoring. This place is self-sustaining for weeks if we need it. Closest city’s three hours out by air.”
Katsuki knew that already. He’d flown in yesterday, touched down on the private landing strip surrounded by nothing but cliffside and jungle. The dorm building had been simple—clean, quiet, and isolated. Just a handful of other staff scattered through the separate housing wings. No luxury. No distractions.
Just the ocean. Just the work.
Exactly what he wanted.
They reached the uppermost level—partially enclosed, partially exposed to open sky. The sun hit his face, warm and sharp in the sudden brightness. The reef stretched below in dazzling color—bright corals, clear shallows, and a deep blue shelf that dropped off into open sea. The outdoor pool jutted into the cove like a crescent, blending perfectly with the reef wall. There were no fences. No nets.
“Damn,” Katsuki muttered.
Aizawa glanced over. “Yeah. It’s a view.”
They stood there in silence for a moment. Below, something broke the surface—barely a ripple. Katsuki caught a flash of silver before it vanished again. It could’ve been a fish. Or not.
“This place,” Katsuki said finally, voice low, “it’s not about containment, is it?”
“No,” Aizawa said. “It’s about trust.”
That word sat with him. Heavy.
Katsuki didn’t say anything else.
The lab smelled faintly of brine and bleach.
Katsuki stepped inside, eyes scanning the rows of equipment. Nothing looked out of place—everything was sleek, clean, and top-tier. Chromatography rigs, nutrient filtration test stations, hydrothermal condition regulators—all things he’d worked with before, though never at this scale. Yuuei didn’t skimp on its tech.
A lanky guy in a lab coat looked up from a tablet near the back counter, bright-eyed and grinning like they weren’t standing in one of the most secure research labs in the world.
“You’re the new guy, huh?” he said, setting the tablet down and extending a hand. “Sero Hanta. I’ve been handling this lab’s diet tracking rotation for the past six months.”
Katsuki gave a firm shake. “Bakugou.”
“Yeah, I heard. Kind of a big deal getting brought on directly into the field team. No one knows how you pulled that off.”
Katsuki bristled faintly. “By being better than the other applicants.”
Sero snorted. “Guess we’ll see. Anyway, welcome to Lab Three, also known as the Tank Chow Lab.”
He waved an arm toward the counters lined with nutrient gels, algae cultures, and sealed vials of preserved fish matter.
“Right now, we’re running trials on two diet patterns—coastal vs. pelagic. Trying to see if recovery rates change depending on the primary source of protein, especially when it comes to injured or immunocompromised merfolk. You’re starting with the quarantine tanks. Light work. Just water sampling, intake tracking, and baseline environment monitoring. You won’t get face time with the reef folks until you prove you’re not going to screw up the chemical balance.”
“I don’t screw up,” Katsuki muttered, but grabbed a clipboard all the same as Sero passed him one.
They made their way out of the lab and down a shorter hallway toward the quarantine wing. Here, things were quieter. Lights dimmer. The water tanks had thicker glass and fewer access points.
“First tank’s empty,” Sero said, tapping his badge to unlock the access panel. “The last patient got moved back to the reef last week.”
The tank was a shallower, circular pool mimicking brackish tidal conditions. Algae still clung to the lower edges, and tiny shrimp darted through the artificial kelp. Katsuki took a few samples, noting the saline levels and minor biofilm development. Nothing unusual.
Then they moved to the next room.
Katsuki recognized the space the moment he stepped in—this was where he’d first seen the flicker of movement, and where he had met Aizawa. The tank here was larger than the last, deeper and colder, ringed with rock ledges and small caverns near the base.
Sero moved casually, prepping the sample tubes and kneeling at the side of the tank near the testing rig. “Grab me a salinity strip, would you?”
Katsuki pulled one from the case and handed it over just as a soft splash broke the surface.
He glanced up—and froze.
A head broke water near the far ledge. Long, red and white two-tone hair clung wetly to porcelain skin. one eye—dark gray—peered at them beneath a damp fringe. The other side of his face, from temple to cheekbone, was roughened by old scar tissue that circled around an icy blue eye.
They stared at each other.
Katsuki held the strip loosely, but his focus was entirely locked on the figure in the water. The mer’s upper body was lean but strong, muscled in that swimmer’s way, and barely visible above the surface. No elaborate jewelry or external markings. Just scarred, quiet stillness—and scrutiny.
“Oh, hey,” Sero said, not missing a beat. “Didn’t realize you were up. Shouto, this is Bakugou Katsuki. He’s taking over my side of the lab rotation for a while.”
The mer—Shouto—didn’t blink. He didn’t respond, either.
Katsuki narrowed his eyes slightly. “Can he even speak?”
“He can,” Sero said easily, taking a water sample without looking up. “He just doesn’t bother unless he wants to.”
A moment passed.
Then Shouto tilted his head the faintest degree. “Interesting,” he said, voice soft but perfectly clear, the syllables sliding together like water over stone.
Katsuki’s brows snapped down. “The hell’s that supposed to mean?”
Shouto’s lips twitched—just barely. The closest thing to a smile Katsuki had ever seen on a Mer, not that he had had the privilege to get this close to one to begin with.
And then, just like that, he sank below the surface again without a splash, vanishing into the deeper shadows near the ledge.
Katsuki glared at the empty water.
Sero chuckled beside him. “Well, that went better than I expected.”
“What exactly was that?”
“Shouto’s been quarantined for a few weeks,” Sero explained, capping a sample. “Caught a secondary infection on his old burn wound. Nothing serious now, but we’re keeping him isolated until the tissue fully reknits.”
“He’s—" Katsuki shook his head. “He’s weird.”
“Oh, totally. But they all are, in their own ways. I think he likes you.”
“Bullshit.”
“Nope,” Sero said with a grin, standing. “I’ve seen him ignore full tours of researchers and interns without even blinking. You? He spoke. Smiled. You should be honored.”
Katsuki muttered something unrepeatable and turned back toward the tank.
Sero nudged him on the way out. “I see why they picked you.”
Katsuki gave him a sideways glare. “What the hell does that mean?”
“You’ve got the energy,” Sero said simply, smiling. “You don’t flinch. They like that.”
Katsuki didn’t reply. He just glanced once more at the water behind him.
Something about that stupid little smile stuck with him longer than it should have.
Katsuki hadn’t been wrong—he was better than the other applicants.
In the first week alone, he cleared his rotation assignments ahead of schedule every day. Habitat notes, water quality logs, diet charts, prey variety and digestion analysis—done and triple-checked. The only part that gave him any real trouble was the data input system. Yuuei didn’t use public software. Every terminal required two-factor clearance, biometric keypads, and redundant password encryptions Katsuki had to memorize like his damn life depended on it.
Which, he supposed, it kind of did.
He’d spent three hours the second night just trying to figure out how to input his own credentials without triggering an alert. Sero had found him hunched over the desk, muttering curses and punching the keys with enough frustration to nearly short the interface. The next morning, one of the security techs casually handed him a cheat sheet labeled "For the Angry One." Katsuki nearly threw it in the tank.
But once he figured it out, he was flying.
Even Aizawa, who barely acknowledged people outside his own rotation, had grunted a vague approval when Katsuki submitted a full data packet two days ahead of schedule.
“Efficient,” was all he said.
But Katsuki knew that was about as high as praise got from a guy like him.
The quarantine tanks became his regular beat. He didn’t mind it. The space was quiet, isolated. Fewer people. Fewer questions. Just the rhythmic churn of filtered seawater, the hum of machines, and—
“Are you always this tense?”
Katsuki flinched so hard he almost dropped the scanner.
He whipped around to glare at the source.
Shouto was floating near the edge of the tank, arms crossed loosely on the ledge, head resting on them as he stared up at Katsuki with that calm, unreadable face of his.
“I’m not tense,” Katsuki snapped.
“Mm,” Shouto hummed, unbothered. “You flinch like you’re expecting to be stabbed.”
“I don’t,” he gritted out, tapping the side of the scanner harder than necessary. “You’re just quiet. And annoying.”
“I wasn’t talking.”
“You exist.”
Shouto tilted his head, silver hair drifting with the current. “You sound very certain that bothers you.”
Katsuki clenched his jaw, eyes locked on the sample readings instead of the smug shape beside him.
This was becoming a pattern.
At first, their interactions had been brief. Shouto surfacing to watch silently, sometimes following Katsuki’s movements from a distance like some unnervingly elegant predator. But over the last few days, he’d grown… bold. Floating closer. Speaking more. Making dry comments Katsuki couldn’t tell were jokes or genuine observations.
And somehow—somehow—it always got under his skin.
Today, for example, Katsuki had been testing the trace mineral levels in the tank sediment when he noticed the shadow moving beneath him. Next thing he knew, a flick of a tail sent a wave sloshing directly over the ledge, soaking his front up to the elbows.
Shouto’s face broke the surface a moment later, absolutely innocent.
“I didn’t see you there,” he’d said, monotone. “You should wear brighter colors.”
Katsuki had nearly dropped his tablet in the water.
Now, with salt drying on his sleeves and a second shift of data entry ahead of him, he was already at the edge of patience.
“What the hell do you even want?” he asked, turning on Shouto again. “You’re supposed to be recovering. Not bugging the staff.”
Shouto blinked slowly. “I like patterns.”
“What?”
“You come in at the same time every morning. You always go to the testing station first. You check the ledge sediment counterclockwise. You scowl more on Tuesdays. It’s interesting.”
Katsuki’s mouth fell open, caught between What the fuck? and Is this guy studying me?
“You’re a freak,” he muttered, jabbing a button on the scanner.
Shouto smiled.
It was small, infuriating, and just amused enough to feel earned.
“I’ve been called worse,” he said, and drifted lazily away into the deeper water, vanishing beneath the low shelf near the center.
Katsuki stared after him for a beat, jaw tight.
He wasn’t sure what was more frustrating—the constant distraction or the fact that he was starting to expect it. That some tiny, quiet part of his brain was waiting for Shouto to appear, every damn morning like a ghost with too much curiosity.
By the end of the week, Katsuki had learned three things:
- Shouto liked cold water and rarely surfaced near heat vents.
- Shouto absolutely did things to piss him off on purpose.
- Shouto was never wrong about his observations, which somehow made it worse.
Once, he’d called out across the tank, “You clench your jaw when you think no one’s looking.”
Katsuki had flipped him off so hard his wrist popped.
Later, Sero passed him in the hallway, smirking. “You two are really hitting it off, huh?”
Katsuki had nearly broken a beaker.
“I am working,” he said through gritted teeth.
Sero just gave a casual shrug. “So’s he.”
Katsuki didn’t respond. He wasn’t sure what would come out if he did.
Katsuki had just finished logging his morning sample data when his comm unit buzzed.
Aizawa (Admin):
Meet me in the upper office. Now.
Katsuki blinked at the message. No explanation, no context—just now.
He double-checked his entries, filed the scanner, and took the elevator up.
Aizawa was already waiting by the reinforced door that led to the admin wing, arms crossed and eyes unreadable as always. He didn’t bother with greetings.
“Your adjustment period’s over,” he said bluntly. “You’re getting upgraded to B-level clearance. From here on out, you’ll have access to observation assignments in the social sector.”
Katsuki blinked. “That fast?”
“You didn’t screw anything up. That’s a rare qualification.”
Katsuki snorted but nodded. He was damn good at what he did—but still, this place didn’t hand out trust easily. The upgrade meant more than just new areas. It meant real access. Unscripted behavior. Interaction.
It meant he was finally allowed to get close.
Back in Lab Three, Sero let out a low whistle when he saw the badge update flash on Katsuki’s console.
“Well, look at you, hotshot,” he said. “Didn’t even blow anything up first. I’m impressed.”
Katsuki rolled his eyes. “You gonna show me where I’m going, or stand there grinning like an idiot?”
“Follow me, o chosen one.”
They wound through the corridor past the upper testing floors and emerged into a short hall that ended in a heavy sliding door with retinal scan. Sero tapped his badge, and the door slid open with a hiss of pressurized air.
“Welcome to the Hub.”
Katsuki stepped inside and froze.
The room was massive. Almost the size of a gymnasium, with walls made mostly of reinforced glass and soft, natural lighting filtering down through panels in the ceiling. But the centerpiece was the tank—it took up nearly the whole floor, stretching from wall to wall in a giant crescent. At one end, a sloped beach-entry pool let water lap up against synthetic sand, and at the other, the far curve of the tank disappeared beyond the building, out into the open ocean.
The ocean.
This was the connection point. This was where the Merfolk came and went, free as tide and wind.
“Holy shit,” Katsuki muttered.
Sero just smiled.
A few people sat around a metal table near the inner wall, laughing and eating breakfast like it was the most casual thing in the world.
“Isn’t that a contamination risk?” Katsuki asked, frowning. “Open food in a work zone?”
“They’re the social group,” Sero said, waving off the concern. “They practically live here. Most of the Mers know them by name. They’re behavior specialists, marine biologists, and a couple of behavioral psychologists thrown in. All cross-trained.”
He led Katsuki over to the table.
The group looked up with easy smiles as they approached.
“Guys, this is the infamous Bakugou Katsuki. Fresh from the quarantine beat and now officially one of us.”
Uraraka Ochaco grinned and stood to shake his hand first. “Hi! Welcome to the real fun.”
Shinsou Hitoshi gave a lazy wave and sipped his coffee. “Hope you’re a morning person. The reef regulars aren’t.”
Tsuyu Asui offered a polite nod. “Let us know if you need anything. The Mers like consistency.”
Then the redhead—Kirishima Eijirou, according to the name stitched on his suit—beamed like sunshine.
“Dude, I’ve heard about you,” he said. “Can’t believe you survived adjustment period in only a week. You’re tougher than you look!”
Before Katsuki could retort, Kirishima zipped up the top of his sleek black wetsuit, adjusted his swim band, and jogged over to the edge of the pool.
“Don’t you—hey!”
Katsuki’s shout was too late—Kirishima launched into the air with a whoop and cannonballed straight into the water, a huge splash erupting over the side.
Katsuki stared, mouth agape. “What the hell was that?!”
Everyone at the table just laughed.
“You’ll get used to that,” Uraraka said, stretching. “Kiri likes to make an entrance.”
“That’s not a study method, that’s a tidal wave!”
“Actually,” Shinsou said lazily, “it’s part of his rapport strategy. He’s catalogued more natural play behaviors than any of us. Merfolk are social, especially juveniles. They bond over activity.”
Katsuki turned back toward the pool where Kirishima was treading water, waving up at him with a grin.
“What about safety protocols?” he snapped.
Tsuyu pointed to the overhead scanner array. “All movement is tracked. Emergency protocols are wired into our suits. Heart rate spikes, oxygen dips, blood detection—it’s all logged instantly. If something goes wrong, admin is alerted before we even hit the button.”
“And most of the Mers who come through here are frequent visitors,” Sero added. “They recognize us. Some even mimic our behaviors.”
Katsuki frowned, still watching the redhead now doing lazy backstrokes across the tank.
This wasn’t research the way he was used to. This wasn’t observation from behind two inches of glass. This was personal. Risky. Raw.
But… honest.
And something about that made his chest feel tight. Like the air smelled different here.
Maybe it did.
Maybe that was the whole damn point.
Sero had left not long after getting Katsuki settled, mumbling something about needing to catch up on backlogged diet entries. That left Katsuki standing awkwardly by the edge of the pool while the rest of the social team zipped up their wetsuits and settled themselves along the tiled ledge like it was just another day at the office.
Katsuki remained dry, arms crossed and frown deepening by the second.
“So,” he muttered, squinting out at the still, glassy water, “where are they?”
“The Mer?” Ochaco said as she rolled up her sleeves. “Probably watching from below. They’re a little wary with new people.”
“They haven’t met you yet,” Tsuyu added. “Give them time.”
“Patience is key,” Shinsou said, then yawned. “They don’t always pop up on cue.”
Katsuki scowled. “I didn’t think this was a petting zoo.”
“Not even close,” Ochaco replied with a grin. “They’re just... cautious. First impressions mean a lot.”
Katsuki opened his mouth to reply—but the water in the center of the tank exploded with a splash.
“—What the—”
He jumped to his feet instinctively, eyes locked on the spot where Kirishima had just been floating. He was gone. The surface frothed for only a second before something burst up again—Kirishima’s red hair, followed by a second head.
Kirishima was laughing, spluttering as he wiped his eyes. The other figure beside him had a shock of bright blond hair, water-slicked and wild, and a toothy grin that could power a lighthouse.
“Got you!!” the newcomer crowed, throwing an arm around Kirishima’s shoulders as they both swam back toward the ledge.
“Bakugou,” Shinsou said dryly, “meet Denki.”
Katsuki stared. The Mer’s upper half looked human enough—lean arms, faint gill lines trailing from behind his jaw—but his lower half was unmistakably not. His tail was long and sinuous, a deep silvery gold, tapering into a pointed fin that flicked with every move. It wasn’t the broad, fan-like tail of the others Katsuki had read about.
It was eel-like.
Dangerous.
“Yo!” Denki said, swimming right up to the edge and planting both arms on the tile. He beamed up at Katsuki. “You must be the new guy! I’ve seen you lurking around the glass. You’ve got a great scowl.”
He held out a dripping hand.
Katsuki just stared at it.
“Don’t touch him,” Shinsou warned with a lazy sigh. “He’s gonna shock you.”
Denki gasped. “You ruined it, man!”
“You always try that on the new guys.”
“Yeah, because it’s funny!”
Katsuki rolled his eyes and stepped back from the edge. “I’m not here to play games with your little aquatic taser.”
Denki grinned wider. “Oh, you’re gonna be so much fun.”
Before Katsuki could answer, movement caught his eye again—this time at the shallow end.
A ripple of soft color moved toward the beach-like slope, and a figure emerged—first hands, then arms, then—
Katsuki’s mouth actually fell open.
A Mer climbed partially up the sand, propping herself on her arms. Her skin was a vibrant pink, glistening in the overhead light, and her long tendrils—not a tail, Katsuki realized with a jolt—twitched curiously. Her eyes were huge and dark, her smile wide.
“Hi!” she chirped. “I’m Mina!”
Katsuki just stared. He’d read about cephalopod variants. Rare. Shy. Nearly impossible to spot outside of deep sea ranges.
He’d never seen one in person.
“I’ve never met a human with a scowl like yours,” she added, blinking her huge eyes. “Come closer, I wanna see it up close!”
Katsuki took a step back. “I’m not in a wetsuit.”
“Aw, boo. Weak excuse.”
A wall of water suddenly surged up from the tank and hit him full in the chest, soaking him from shoulders to knees.
“What the hell?!”
He staggered, coughing, blinking the saltwater from his eyes.
When he finally wiped them clear, he saw him.
Floating just a few feet away, hair fanned out like a halo, face as expressionless as ever.
“Who the hell let your fishy ass out of quarantine?” Katsuki barked.
Shouto tilted his head. “Sero. I’m fully healed now.”
He turned calmly to the others. “Everyone, this is my best friend, Katsuki.”
Katsuki gawked at him. “I am not—!”
“Oh my god,” Ochaco burst out, nearly falling over laughing.
“Best friends already?” Kirishima whooped from the pool.
Tsuyu covered her mouth to hide a smile. “That was fast.”
Katsuki rounded on Shouto, who looked entirely unbothered by the attention. “What is wrong with you?”
Shouto replied with a shrug. “You’re consistent.”
“You’re insane.”
“You’re loud.”
“You’re wet!”
“That is true,” Shouto agreed, and dunked under the water with a graceful roll.
Katsuki stood there, water dripping from his arms, hair plastered to his forehead, and fuming so hard the air might’ve hissed.
The others were absolutely howling.
“Welcome to the team,” Shinsou said with a smirk.
Katsuki stayed in the Hub for the rest of the day, soaked to the bone and silently swearing to bring a towel next time. A wetsuit too, probably—not that he was ready to jump into the damn tank with a bunch of sharp-toothed fish people.
Not yet, anyway.
Integration, they called it.
Translation: sitting still and making himself visible, accessible, and—ideally—non-threatening.
“Just hang out for the day,” Uraraka had said. “Get used to the rhythm. Let them see your face. Some of them are just cautious around new humans.”
So, for the rest of the day, Katsuki stayed poolside. It was unlike any work environment he’d ever been in.
Mer came and went throughout the day, swimming in from the open-water section, or surfacing from the deeper center. Some looked like the classical versions he’d studied—long fins, scaled torsos, powerful tails. Others were clearly different species entirely. Some had luminescent markings. Others had claws instead of nails. One swam by upside-down, chittering at Tsuyu, who just laughed and splashed water back at him.
He spoke with whoever came close enough to engage, made mental notes about who approached first and who kept their distance. He tracked which areas of the tank different Mer seemed to favor. He noticed the natural flow of conversation between his coworkers and the aquatic visitors—easy, warm, like talking with old friends.
At one point, Katsuki spotted Ochaco sitting cross-legged on a dry panel near the shallow beach, holding up waterproof flashcards with letters printed in bold font.
A pair of young Mer watched her closely from the water, brows furrowed as they tried to mimic the sounds.
“A,” she said, tapping the card. “Ah. Like ‘apple.’”
“Ah,” one repeated, voice high and crackly.
Katsuki blinked. That was… not what he expected.
“Language training?” he muttered to Shinsou, who lounged nearby, pretending not to be invested.
“Only for the ones who ask,” Shinsou replied without looking up. “We’re not teaching them for fun. Some of them want to understand more—what we say, what we write. We accommodate when we can.”
Katsuki nodded. That made sense. Kind of.
He was used to studying things. Monitoring variables, controlling samples. But this? This was more like… building relationships.
Later, Kirishima coaxed a more energetic Mer named Hizashi into playing a game of tag underwater, which turned into something between a coordinated acrobatic routine and absolute chaos. Denki got involved halfway through and immediately crashed into a floating puzzle, flipping it over and shouting “I WIN!” even though no one had explained the rules.
Mina swam up the shallow ramp at least three times just to check on Katsuki, flicking water at him playfully before slipping back into the water with a giggle.
He met Momo later that afternoon. Her tail was long, elegant, and carried herself with a kind of grace Katsuki couldn’t describe. She didn’t approach right away—just circled the tank, watching from a distance, golden scales catching the light. She greeted him politely, asked about the algae samples he’d been cataloging, then offered a surprising amount of insight into local current shifts affecting microfauna growth near the reef mouth.
She drifted off after a bit, but something about it felt deliberate. Like an assessment he’d passed. Katsuki had to remind himself that many of the Mer weren’t just intelligent. They were aware. Curious. Capable of learning and adapting—sometimes faster than humans.
A little later, another Mer—smaller, with dark purple markings and narrow fins—popped up near the deeper side and watched him with sharp eyes. She didn’t speak at first, but after several minutes of silence, she simply muttered, “You’re loud even when you’re quiet,” and ducked back underwater before he could respond.
Shinsou had smirked from where he sat nearby. “That was Jirou. That’s basically her version of saying hi.”
Katsuki rolled his eyes.
Not all interactions were easy. Some Mer simply swam by without so much as a glance. A few vanished the moment he stepped near the ledge. But no one reacted with fear. Just... caution.
Shouto came and went throughout the day, slipping in and out of the water like he lived between worlds. He never stayed long—just surfaced briefly, said a single cryptic thing, and disappeared again. At one point, he drifted by and said, “Mina thinks you look like a startled rockfish.” Then left.
Katsuki hadn’t even dignified it with a response.
“You’re staring again,” Katsuki muttered around midafternoon as Shouto surfaced beside him for the fourth time that day.
“I’m not,” Shouto said flatly. “I’m observing.”
“Same difference.”
Shouto blinked up at him, deadpan. “Izuku is curious about you.”
That caught Katsuki off guard. “Who?”
Kirishima, who had just pulled himself out of the water to grab a protein bar, gasped.
“Izuku?! Izuku saw you?!”
Katsuki turned slowly to him, one brow raised. “...What?”
“That’s insane!” Kirishima beamed, towel around his shoulders. “Izuku never comes this close to the hub. He’s one of the most reclusive Mer we’ve ever worked with.”
“Is he part of the rehab program?”
“Kind of,” Shinsou offered from where he was jotting something into a waterproof tablet nearby. “He’s not injured or under observation. Just... doesn’t usually like people. Comes in through the reef sometimes. Watches from below.”
Katsuki frowned. “So he didn’t even talk to me?”
“Nope,” Kirishima grinned. “But if he swam close enough to see you, that’s huge. He’s curious. That’s the first step.”
“I didn’t even see him,” Katsuki muttered, eyes narrowing as he glanced toward the shadowed stretch of the pool that led into the ocean.
“You probably wouldn’t,” Shouto said simply. “He’s good at hiding.”
“I’ve worked here for around five years now and I only met him once with Aizawa, and that was only because he brought an injured pup to us” Shinsou spoke thoughtfully “He seemed very worried but friendly enough at the time.”
Katsuki didn’t respond.
But he looked longer than he meant to—at the still water, the distant reef, the dark space just beneath the surface.
Curious, huh?
-
Katsuki stood at the edge of the Hub’s beach entry, the morning sun filtering through the skylights and catching on the water’s surface in soft, dancing ripples.
His wetsuit clung tightly to his skin, dark and streamlined, with the Yuuei emblem stitched just above the left shoulder. He rolled his shoulders once, exhaled through his nose, and tried not to look like this was a big deal.
It wasn’t.
Just water. Just a swim.
Just the fact that every Merfolk in the area might be watching him from below.
No pressure.
“Looking sharp,” Kirishima called out, already in his own suit and stretching near the deeper ledge.
“Don’t jinx it,” Katsuki muttered, flexing his fingers as he waited for the others to finish their prep. Ochaco and Tsuyu were already at the edge, chatting with Momo near the equipment lockers. Shinsou was logging their schedule on the wall terminal, half-awake as usual.
Once the others gave the nod, Katsuki stepped forward and slowly descended into the shallow slope of the artificial beach.
The water was cool, not cold—just enough to make him suck in a breath as it reached his waist. It lapped gently at his sides, salt tang lingering in the air. The suit helped with buoyancy, but he still moved with practiced control, making sure not to splash or send disruptive ripples across the pool.
He paused halfway in, crouched slightly with his hands in the water.
And waited.
The others stayed near, close enough to intervene if anything went sideways, but far enough to give him space. This was his moment. His first moment.
He didn’t have to wait long.
A soft splash to his left made him glance up—and Mina was there.
She wasn’t swimming. She’d crawled up the beach slope on her arms like she had before, her vibrant pink skin glistening and her large, curious eyes locked on him.
“Hi!” she said brightly, her grin almost too wide for her face.
Katsuki gave a stiff nod. “Hi.”
Mina leaned forward, her tentacles trailing behind her like flowing ribbons. She crept closer, stopping only a foot away before planting both hands on his chest.
Katsuki froze.
“Huh,” she said, pressing her palms flat. “Your skin’s rougher than I thought.”
“I’m wearing a wetsuit,” he deadpanned.
“Oh! Yeah, obviously. What’s this?” she asked, tugging at one of the reinforced seams on his arm. “Is this for heat regulation?”
“Mina,” he warned, voice tight.
But she was already running her fingers down his shoulder, tapping at the seal on his gloves, poking curiously at the buckles on his legs.
“You humans and your gadgets,” she muttered in fascination, one of her tentacles flicking against the side of his neck.
Katsuki twitched.
“Mina,” he said again, sharper this time. “I’m not a science exhibit.”
She blinked at him, then giggled and leaned back. “Sorry! You’re just new. And squishy.”
“I am not—” He cut himself off with a sigh and waved her away. “Go swim or something.”
She blew a playful bubble at him before spinning around and diving back into the water with the grace of a seal.
With her gone, Katsuki finally submerged fully and pushed away from the slope, letting the water take him.
The pool felt bigger in it. Endless. Quieter than he expected, the pressure of the water dulling the sounds of the world above. He swam in slow, careful strokes, keeping his profile low and steady. He passed through shafts of filtered light, glided over the rock formations and coral structures near the edge, and didn’t speak unless someone approached him first.
It took longer for the others to come near.
Unlike when he’d been on land, here in their space, the Merfolk were more reserved. He could feel their eyes on him—passing shadows, sudden ripples—but they didn’t close in right away.
He respected that.
“Heya.”
A voice chirped near his ear, far too close.
Katsuki spun
“AGHH! WHAT THE—”
Katsuki shot backward, flailing his arm with a curse loud enough to echo off the walls.
Denki surfaced a second later, absolutely howling with laughter.
“Oh man, you should’ve seen your face—!”
Katsuki rounded on him, water flying from his hair. “I will fry you, you electric bastard—!”
“It was just a little jolt!” Denki wheezed, doing a barrel roll in the water. “Barely even registered on the sensors!”
“I’M GONNA REGISTER YOU AS A HAZARD—”
“Katsuki!” Ochaco called gently from the ledge, barely hiding her smile. “It’s a compliment! Denki only shocks people he likes.”
“I’ll like him right into a containment tank—!”
Denki blew him a kiss before vanishing back below the surface with a flick of his sleek, eel-like tail.
Katsuki fumed.
After swimming around the Hub tank for hours, Katsuki was surprised by how natural it began to feel. The buoyancy. The water. The way his coworkers drifted in and out of conversation both above and below the waterline like it was second nature.
He was pulled into a few games—Denki tried to teach him how to race backwards, which ended in Katsuki nearly crashing into a reef cluster—and got grabbed by Mina more times than he could count. Kirishima had taught him a few underwater signs the Mers used with the team, and Shouto made exactly three appearances, each with no explanation and no greeting before disappearing again.
It was… exhausting. And oddly fun.
They’d insisted he eat lunch with them on the deck near the shallows, handing him a tray and practically dragging him to the round table.
“You’re one of us now,” Kirishima said with a grin, his hair still damp and curling at the tips. “You sit, you eat, and you bitch with the rest of us.”
Katsuki had grumbled but obeyed.
The food was better than expected. The conversation was loud and full of teasing.
And then, after dishes were cleared and assignments were passed around, Sero arrived with a mesh crate of oxygen tanks.
“Alright, back to your real job,” he said, grinning. “Time to dive.”
Shinsou was already halfway into his gear by the lockers, slipping the final strap into place. Katsuki suited up alongside them—oxygen tank, regulator, reinforced gloves, and a smaller toolkit strapped to his thigh for samples.
They checked each other’s gear once, then again.
And then—down they went.
The Hub tank was vast, but descending to the bottom revealed a whole different world.
Light filtered down in dappled shafts, painting the rock faces and swaying sea grass in hazy gold. Artificial reef clusters teemed with life, and the water was alive with the hum of current and the low mechanical pulse of the distant reef gates. It was quiet—peaceful—but not empty.
Katsuki spotted a few Merfolk lingering near the reef wall, watching from a distance as he and the others worked.
They didn’t approach. Didn’t interrupt. Just watched.
He kept focused, using a small suction core to collect sediment samples, carefully scraping a few clusters of biofilm for lab testing, and checking the temperature and pH sensors embedded along the tank floor.
It was meticulous work.
Familiar.
And for the first time since he arrived, Katsuki felt that edge of comfort that came with doing what he knew he was good at.
Sero floated a few meters away, scanning an artificial shell cluster while Shinsou logged notes on a submersible tablet.
Katsuki paused, flexed his hand, and stretched his shoulders—waiting for the others to finish.
A bump from the side.
Gentle, but enough to shift him mid-float.
He turned, half-expecting Denki again—but instead, he saw the glint of a red and white tail sweeping lazily through the water. Shouto.
Of course it was Shouto.
Katsuki immediately flipped him off.
Shouto didn’t even look back as he glided into a tall, thick patch of seagrass swaying in the current. Just vanished between the stalks like he’d never been there.
Katsuki rolled his eyes and was about to look away—when he saw them.
Eyes.
Bright green. Wide. Watching him.
They blinked once from the shadowed interior of the seagrass. Not fearful. Not threatening. Just... curious.
Katsuki held still.
He didn’t move. Didn’t blink. Didn’t breathe, even though the tank kept his oxygen steady.
Just a face—young, maybe? Soft features. Tousled green hair, trailing behind a pair of fins that framed each cheek. He couldn’t see the body, but he caught the shimmer of green scales for half a second. He doesn’t think he would have even spotted the Mer if Shouto hadn’t swam that way.
And then—gone.
The Mer vanished deeper into the grass, quick as a shadow fleeing the light.
Katsuki blinked hard, heart suddenly hammering faster than before.
He stayed there, scanning the patch, eyes locked on the place he’d seen him.
Nothing moved.
Just the grass. Just the quiet.
Then a nudge to his arm.
Katsuki turned—Shinsou hovered beside him, lifting his wrist and tapping the ascend signal.
Their time was up.
He gave one last glance toward the grass.
Still nothing.
And then he kicked upward, lungs full but chest tight with something else entirely.
The light hit his face the second he broke the surface.
Katsuki pulled out his mouthpiece with a breath, blinking against the sudden brightness after so long in the water. He drifted toward the pool's edge, pushing himself up onto the platform with practiced ease. Water streamed off his suit in sheets, pooling at his feet as he tugged off the gloves and shrugged out of the oxygen tank.
Shinsou surfaced behind him and climbed out more slowly, setting his gear down with a soft clunk. Sero was still down below, logging the last of their samples, but Katsuki hardly noticed.
His mind was somewhere else entirely.
He peeled off the top half of his wetsuit, shaking water from his hair. Then, as casually as he could manage, he asked, “Hey… is there a Mer with green hair? And green scales?”
Shinsou paused mid-unhooking his mask.
Didn’t move. Didn’t look at him. Just… froze.
Katsuki frowned. “What?”
Shinsou turned slowly and gave him a look that was equal parts interest and caution. “Why?”
Katsuki hesitated, then leaned against the ledge, arms crossed. “Saw someone in the seagrass while we were collecting samples. Just for a second. Large frame, green hair, big eyes. I don’t think I’ve met them yet.”
Shinsou let out a quiet breath and set his gear aside.
“That… was probably Izuku.”
Katsuki blinked. “Seriously?”
“Unless there’s a new Mer in the area, yeah. And if that were the case, one of the regulars would've told us.” He ran a towel through his dripping hair. “They’re protective like that.”
Katsuki glanced back at the water, toward the edge of the reef zone.
“He didn’t do anything. Just watched. Then left.”
“Sounds like him,” Shinsou muttered.
Katsuki turned back. “Okay, but why? Why the hell would he be watching me?”
Shinsou gave him a long, thoughtful look, then sat down on the platform, feet still in the water.
“Honestly? I don’t know.”
Katsuki narrowed his eyes. “That’s not helpful.”
“I mean it,” Shinsou said. “Izuku’s hard to figure out. He’s not aggressive. Not scared, either. But he doesn’t like crowds. Doesn’t interact with the staff. Hell, we usually go months without seeing him up close.”
Katsuki remembered the flash of green hair. The wide eyes, curious and unblinking.
“I’ve been here less than two weeks,” he muttered.
“Exactly. And he’s already visited the Hub more times than he has in the last three months combined. You haven’t spoken to him. You’ve barely seen him. And yet, he’s following your movements. Watching from the reef. Getting close while you’re distracted.”
“That’s not creepy at all,” Katsuki said dryly.
Shinsou shrugged. “Merfolk aren’t like us. Their version of curiosity looks different. Less direct. They test boundaries first. Watch how you move. How you treat others. How you react when they’re nearby.”
“Sounds like stalking.”
“Sounds like assessment,” Shinsou corrected. “It’s how they’ve survived this long. Especially someone like Izuku. We don’t know everything about his past, but it’s clear he’s… cautious.”
Katsuki was quiet for a moment, watching the sunlight glint on the water’s surface.
“He didn’t look scared,” he said.
“No,” Shinsou agreed. “He’s not scared. He’s interested. And that’s the weird part.”
Katsuki raised an eyebrow.
“He doesn’t show interest in people,” Shinsou went on. “Not even in the presence of other Mers. I’ve seen him interact maybe twice. And only when he thought no one was watching.”
Katsuki frowned, arms crossed tighter. “So what, I’m some kind of science project to him?”
Shinsou gave a slow, amused smile. “Welcome to the feeling. Now you know what it’s like when you’re the one being studied.”
“Tch.”
They sat in silence for a moment, the only sounds the hum of the filtration system and the distant chatter from the upper deck.
“You’re taking it better than I thought you would,” Shinsou said finally.
“I’m not,” Katsuki muttered.
But he was lying.
Because deep down, something about that flash of green hair and those wide, unblinking eyes wouldn’t leave his mind. It lingered, like the memory of a current tugging at his feet in calm water—gentle, but impossible to ignore.
Katsuki had always been one for routine—his days at Yuuei had settled into a rhythm that didn’t dull with repetition. If anything, it sharpened his focus.
Mornings belonged to the lab.
He’d get in early—before most of the team was even awake—and spend the first few hours running diagnostics, logging sediment analyses, testing nutrient balances, and calibrating the internal sensors from the previous day’s samples. Sero would usually wander in by mid-morning, still chewing on a protein bar and cracking jokes about Katsuki trying to marry the data.
At noon, they’d switch.
Sero took over the lab, and Katsuki would head up to the Hub to eat and decompress.
He didn’t fight it anymore when the others insisted he sit with them. Kirishima always made a spot for him, and Tsuyu would often pass him part of her fruit without asking. Shinsou only grunted his greetings unless caffeinated, and Uraraka had a habit of asking what he’d learned each morning as if they were trading discoveries like cards.
After lunch, he’d spend an hour or two in the water—sometimes just floating, sometimes joining behavioral observation tasks—before gearing back up to meet Sero again for afternoon sampling rounds across the facility’s other tanks.
The only area still off-limits to him was the nursery.
He didn’t ask about it. Didn’t have to. The security protocols were tighter, and it was clear from the way everyone talked that only senior staff like Aizawa had clearance for that wing. It didn’t bother him.
He had plenty of other things to focus on.
Like the Mers.
The longer he stayed, the more he realized how little his academic background had actually taught him.
There were dozens of types that visited the Hub—some familiar, some so rare Katsuki had only ever read about them in scattered journal footnotes. Shark-types with jagged black tails and razor smiles. Coral-colored reef dwellers with ornate fin patterns. Deep-sea variants who came only at dusk, glowing faintly in the darker corners of the tank.
But it wasn’t just the biology that caught him off guard.
It was the culture.
The complexity.
They weren’t just intelligent—they had a society. An intricate web of relationships, hierarchies, personal boundaries, and customs that evolved with region, species, and even mood.
He’d learned more in a month watching them than in four years of graduate-level research.
Uraraka had explained it best one afternoon while drying off on the edge of the pool.
“The reef out there,” she said, nodding toward the ocean beyond the open wall, “is massive. It stretches for miles past the drop-off. Most of the Mer that visit us live out there. Families, pods, traveling groups—it’s a whole world under the water. We’re just a dot on the map.”
Katsuki had listened, quietly absorbing it all.
“They don’t always interact with us,” she continued. “But they talk to each other. That’s how they know what’s happening here—who’s coming and going, who’s been injured, who’s staying. News spreads fast in the water.”
It explained how new Mers were accounted for even if they never came inside the facility. How one Mer could visit only once and still be greeted by name weeks later.
They were constantly in touch.
Constantly watching.
And apparently, one of them would not shut up about him.
Shouto had become relentless.
Every time Katsuki so much as dipped his foot into the pool, Shouto would appear—gliding up beside him like a ghost and speaking in that maddeningly calm voice.
“Izuku was here yesterday,” he’d say casually. “He watched you scrape barnacles for twenty minutes.”
Or:
“He likes the way you swim. Says you don’t thrash as much as other humans.”
Or the worst one yet:
“He thinks your eyebrows are expressive.”
Katsuki nearly drowned himself on that one.
“Will you shut up about him already?” he barked one afternoon after Shouto had once again surfaced just to pass along some unnecessary commentary.
“I’m just relaying what he says,” Shouto replied, utterly unbothered. “We’re friends. I thought you’d want to know.”
“I don’t!”
“Hmm.” A pause. “You’re scowling more than usual. He’d probably think that’s cute.”
Katsuki lunged, but Shouto had already vanished in a flick of his koi-like tail, trailing smug bubbles behind him.
It was ridiculous.
Infuriating.
Distracting.
And yet…
Every time Katsuki stepped into the water now, he found himself scanning the edges of the reef. Every time he floated near the seagrass or passed the tall stone ridges at the far wall, he checked—just briefly—for green.
He told himself it was just professional curiosity.
After all, if one of the most elusive Mers in the entire reef system was interested in him, that had to mean something worth documenting.
Katsuki arrived earlier than usual.
The hallway lights still hummed low in their morning setting, and the Hub was quiet when he stepped through the sliding door. Only one other person was there—Tsu, typing away at the table with her laptop and a mug of something steaming at her side.
She wasn’t in her wetsuit yet, just a loose hoodie and shorts, legs tucked up beneath her as she tapped at the keys with patient focus.
“Morning,” she said softly, without looking up.
Katsuki nodded, still rubbing a towel through his damp hair from his earlier shower. “Mornin’ ”
She’d insisted he call her Tsu after the first week. Said Tsuyu was too formal and Asui made her feel old.
He didn’t argue. She was quiet, but sharper than most of the team, and had a way of watching people like she saw things they didn’t.
He dropped his gear by the lockers, slipped off his shoes, and made his way to the edge of the pool.
The water was still. Untouched.
No one else was here yet.
Katsuki sat down on the ledge and dipped his feet in. The water was cool, soothing, the morning light casting soft reflections across the surface.
He didn’t say anything as Tsu kept typing.
They were both used to silence by now.
Minutes passed. The weight of the water lapped gently against his ankles.
Then the sound of Tsu’s chair shifting caught his attention.
“I’ll be back in a minute,” she said, standing and closing the lid of her laptop. “Gotta put this away.”
Katsuki grunted in response, not turning his head as she padded out of the room.
And just like that, it was quiet again.
He stared out across the water, letting his thoughts wander.
Until a soft splash broke the silence. Barely there.
But Katsuki heard it.
He straightened instinctively, eyes scanning the surface.
And froze.
Across the pool, just inside the hub, a head had broken the water.
Not Denki’s wild blond curls.
Not Mina’s vivid pink hair.
Not even Shouto’s split tones.
This was green. Deep, mossy green, plastered to pale skin, water trailing down sharp cheekbones and wide, unreadable eyes.
Izuku.
Katsuki’s breath caught in his throat.
He didn’t move. Didn’t speak.
He just stared as those green eyes locked with his across the water.
The Mer didn’t blink.
Neither did he.
The silence stretched long, like a held breath.
Katsuki couldn’t even hear the filtration system. Just the slow beat of his own pulse in his ears.
And then—movement.
Izuku began to drift closer. Not fast, But not shy, either.
Gliding just beneath the surface, head still up, eyes never leaving Katsuki. His fins barely stirred the water as he cut a quiet path toward the ledge.
Katsuki’s instincts—so often sharp and aggressive—felt different this time.
Like prey spotting a predator.
The closer Izuku came, the more Katsuki noticed—the way his long fingers dragged the surface of the water, the faint shimmer of emerald scales along his shoulders and neck, the flick of a large tail just beneath the surface.
He was halfway to Katsuki now.
Katsuki leaned forward, unconsciously, hands braced on the tile.
He didn’t know what he would say.
Didn’t know if he should say anything.
The door to the hallway opened again.
Tsu’s voice carried in gently. “Okay, now I’m—”
Katsuki whipped his head around.
By the time he turned back—
Gone.
The water was still.
Only the faintest ripple remained, already fading.
He stared at the spot Izuku had just been.
Eyes narrowing. Jaw tight.
“Tch,” he muttered under his breath.
But his heart was racing.
And the ghost of green scales lingered in his mind far longer than they should have.
Katsuki didn’t mention the encounter right away.
He’d mulled it over during his morning swim, tried to make sense of it while the others splashed and laughed and chatted around him like any other day. Izuku hadn’t returned. No green eyes. No shimmer of scales. But Katsuki couldn’t stop thinking about that gaze—direct, unreadable, and far too intense for a Mer who supposedly never surfaced.
Eventually, after lunch, when most of the team was lounging by the shallows and cataloging notes, he dropped it as casually as he could.
“So,” he said, leaning back on his hands, eyes fixed on the water, “Izuku showed up earlier.”
The reaction was immediate.
“Wait—what?!” Ochaco nearly choked on her drink.
“Came to the surface?” Tsuyu asked, eyebrows lifting.
“He was watching you again?” Kirishima leaned forward, grin spreading like wildfire. “Dude. That’s huge.”
“Hold on,” Shinsou cut in, already grabbing his tablet. “What exactly happened?”
Katsuki sighed. “It was early. Tsu had just left to put her laptop away. I was alone for maybe a minute, and I heard the water move. He was across the pool. Just his head above the surface, staring at me. Then he started swimming toward me. Didn’t say anything. Didn’t stop. He was halfway there when Tsu came back in. And then he was gone.”
They stared at him in silence.
Then the chaos began.
“He got that close?!”
“Holy crap, he never comes to the surface—”
“Did he speak at all?”
“No,” Katsuki muttered. “Didn’t even blink.”
Tsuyu was already flipping through notes on her tablet. “That’s significant behavior change. From observation to approach. Especially surface-level engagement.”
Kirishima nodded along, thoughtful for once. “He doesn’t even come this far into the Hub unless he’s injured or following someone.”
“Or testing someone,” Shinsou added.
Ochaco furrowed her brow. “What could’ve triggered the shift?”
“Could be seasonal?”
“Could be interpersonal.”
“Could be you,” Shinsou said, glancing up at Katsuki.
“Don’t start.”
But Shinsou was already moving, flipping open a fresh screen and calling out to Denki, who was floating lazily near the edge of the pool, arms draped over the tile.
“Hey Denks,” he said, voice easy. “You been hearing anything? Izuku’s been acting different.”
The Mer blinked at him with wide, golden eyes. Then slowly—deliberately—he smiled.
And shrugged.
Kirishima narrowed his eyes. “You know something.”
Denki gave a little innocent chirp and kicked backward, tail swishing as he dipped beneath the surface with a sparkle of light.
“Of course he does,” Shinsou muttered. “They’re tighter than we think.”
“Maybe Izuku said something to him,” Ochaco said, already typing. “Denki doesn’t usually keep secrets, but if Izuku asked—”
“They’re Mer,” Tsuyu said gently. “They communicate in ways we don’t always register.”
“And now he’s surfacing to stare at me,” Katsuki muttered, rubbing his temples. “Great.”
Shinsou, already elbow-deep in behavioral patterns and past observation logs, pointed to a screen. “Okay, but let’s think about this. Surface approach, unprompted, during a window when he thought no one else was present? That’s a test. It has to be.”
“You’re not helping.”
“It means he’s escalating,” Tsuyu added. “Curiosity becoming interaction.”
“Or interest becoming intent,” Shinsou countered.
Ochaco raised a brow. “You’re saying he’s making a choice?”
“Maybe. Or maybe he’s just watching to see what Katsuki does next.”
They all looked at him again.
Katsuki stared blankly back.
He didn’t know what half the things they were throwing around meant—bonding signals, pod migration indicators, mutual observation loops (whatever the hell that was). Shinsou and Tsuyu were debating linguistic mimicry as a potential next step. Ochaco was talking about bringing out the flashcards again. Kirishima had grabbed a whiteboard and was drawing charts like this was a full-blown thesis.
Katsuki had had enough.
With a grunt, he stood up, grabbed his towel, and tossed it over his shoulder. “I’m going to the lab.”
Sero was waiting to swap shifts anyway.
The others barely noticed him leave, too wrapped up in their spiraling theories.
Which was fine.
Let them overanalyze.
He just needed to get back to something that made sense.
Like water samples.
Not mysterious Merfolk with hypnotic green eyes who appeared in silence and vanished like ghosts.
Definitely not that.
-
Katsuki hadn't seen the Hub in two days.
Not because he didn’t want to—hell no—but because work had buried him.
The quarantine tanks needed adjusting. One of the larger habitats had to be rebalanced after a new arrival was scheduled, and the chemical levels were off just enough to make it a hazard. A shark Mer named Tetsutetsu had been transferred in—ironically recovering from a shark attack—and Katsuki had been assigned to prep his temporary habitat.
It wasn’t easy. The guy was huge, aggressive even when injured, and required specific mineral levels to support his species' faster healing. Katsuki spent the better part of two days adjusting calcium, salinity, and oxygen saturation—tuning everything down to decimal precision.
And when the transfer was complete, paperwork hit like a second wave.
Full tank diagnostic reports, behavior logs, diet changes—all of it had to be entered before morning rounds.
By the time the second day ended, the sun had long since disappeared, and the labs had emptied out. Most of the social team was probably already at the dorms, maybe eating or passed out in their bunks.
Katsuki checked the last column of his data entry sheet, saw the alert ping for a pending acidity test in Tank Hub-01, and cursed under his breath.
“Of course it’s overdue,” he muttered.
He could wait until morning—but then he'd have to deal with it before his coffee, and there was no chance in hell he was starting his day like that.
So he grabbed a test kit, slung a towel over his shoulder, and headed for the Hub.
The room was dark when he entered, the overhead lights dimmed to their nighttime setting. Only the faint blue glow of the water panels illuminated the floor, and it cast long shadows over the rippling pool.
Katsuki flipped on one of the overhead lights at the side of the deck, just enough to create a soft yellow pool of light that reached a portion of the water.
He didn’t need the whole tank lit.
Just enough to work.
He knelt by the edge of the pool, clicked open his test kit, and got to work—drawing a small sample into a vial, sealing it, marking it, repeating the process with steady, practiced ease.
The silence was almost comforting.
For the first time in two days, he wasn’t being asked to fix anything.
Just finish.
He slid the final vial into its padded case and stood up, stretching his neck with a grunt.
That’s when he heard it.
A figure breaching the water just beyond the edge of the light.
He didn’t turn right away. “Don’t mess with me, Shouto. I’m going to bed.”
No answer.
Katsuki sighed and finally glanced up.
The breath left him in a beat.
Not Shouto.
Not anyone he was used to seeing here after dark.
Izuku.
He was just outside the reach of the overhead light, water gliding over his shoulders, head tilted slightly, gaze steady and unreadable.
Katsuki couldn’t move.
Neither did he.
They just stared at each other in the silence of the empty Hub, with the water gently lapping at the ledge and the shadows stretching long around them.
Izuku was here.
His fingers still rested on the sealed test kit, but he’d forgotten it was even there.
The only sound was the slow breath of water and the faint hum of the lights.
He stared at the Mer just meters away, the water lapping softly around Izuku’s shoulders. The dim lighting left much of his expression in shadow, but his eyes glowed faintly in the low light—green and glassy, unblinking.
Katsuki shifted slightly where he knelt, tension wound tight beneath his skin.
“Alright,” he muttered, voice low, almost wary. “You gonna say something? You’ve been following me around like some fishy ghost since I got here.”
No response.
Just the faintest sound.
A trill. it wasn’t threatening, just strange. Musical. Curious.
Izuku began to move.
Slowly. Smoothly.
The closer he got, the clearer he became beneath the surface. Broad shoulders and lean muscle framed his upper body, shifting with each stroke. His tail was massive—sleek and strong, dark green fading to black toward the edges of his fins, which trailed behind him like ribbons. Scale patterns curled down his arms, black streaks glinting as they ran from shoulder to wrist.
He stopped just in front of where Katsuki knelt.
Katsuki didn’t know what to say. He didn’t even know if Izuku could speak.
That unreadable look was still on his face—something between curiosity and calculation. Maybe awe. Maybe challenge.
The lights flickered.
Barely more than a blink.
But Katsuki looked up—just for a second.
And in that second, everything changed.
There was a splash. A shift in pressure.
Then pain.
Sharp and sudden, teeth sinking into the meat of his forearm just below the elbow.
“AGH! What the fuck!”
Katsuki shouted, jerking back with a panicked curse, falling hard onto the tile as Izuku released him and retreated in a blur of motion.
Water churned where he’d been.
By the time Katsuki scrambled back to his knees, clutching his arm, Izuku had already backed away into the deeper center of the pool—hovering, watching.
Blood ran down Katsuki’s arm in thick rivulets, soaking the edge of his sleeve and dripping onto the tile below. He clutched the wound to his chest, snarling.
“What the hell is wrong with you?!” he barked, breath shallow. “You bit me, you lunatic sea freak!”
Izuku didn’t move.
Didn’t answer.
Just stayed there—eyes wide, expression unreadable, tail shifting slowly behind him as though he hadn’t just attacked.
Katsuki backed up fast, pressing a hand over the bite as he ripped off his shirt with the other, using it to staunch the bleeding. The wound was deep—too deep for comfort—but the flow was already slowing.
Still, it hurt like hell.
He glared at Izuku one more time, seething. “You know what? Screw this.”
One hand and keeping pressure on his arm with the other, he staggered up, blood smearing across his chest and dripping onto the deck.
Katsuki stormed out of the Hub.
The lab lights were too bright when he stepped inside, blood drying sticky against his skin. He yanked open the emergency med kit, cursed through gritted teeth as he rinsed the wound under the sink, and hissed as disinfectant met broken flesh.
The bite had clear indentations. Sharp. Clean. Precise.
Definitely not a mistake.
He wrapped it tightly, the gauze binding hot against his skin, then tossed the ruined shirt into the biowaste bin with a snarl.
“Crazy-ass fish,” he muttered, chest still tight.
He checked the clock.
Late.
Too late to go back and deal with anything else.
The samples could wait.
He turned off the lab lights, still flexing his fingers beneath the bandages, and made his way toward the dorms.
But even as he climbed into bed, arm throbbing, adrenaline crashing, one question refused to leave his head:
Why the hell did he bite him?