Chapter Text
He smelled like chili peppers. Not fire; not burning oil. If there was anything consistently detectable beyond a heap of capsaicin analog, linalool, bell pepper pyrazine, and a hint of methyl salicylate in his base scent, then Senku Ishigami smelled like rayu sauce. An undertone of sesame oil if he was in a good mood, something umami in there that could be garlic if you let your imagination run wild. There was occasionally a crisper scent when he was in a certain mood, but if anyone dared mention that he smelled a bit like spring onions when his cycle hit they were asking for retribution. Peppers got the crisp, fresh part of their scent from cucumber aldehyde, and so did he. He’d done the chemistry himself after keeping a handkerchief under his uniform top to soak up the sweat and oils for a month.
The scents of Alphas, Omegas, and Betas followed simple rules, but of course there where outliers. Biology was messier than the average person assumed. Gender and sex expression were a spectrum, even if most people got stuck on grade school level biology and never knew that there were twelve gene markers across the sex chromosomes with fifty or so variants each that determine someone’s secondary sex traits. Still, there were general rules of thumb: Mineral and musky scents were for alphas, with some unfortunate cheesy or even alcoholic fermented scents. Senku knew one alpha from school who smelled a lot like week-old cabbage with a bloody metallic undertone, that poor idiot would have to find someone with very specific taste or figure out a cologne that would spice it into something closer to kimchi. Betas could smell like absolutely anything, but had fainter base scents that were easily overrun by whatever emotion their scent glands were broadcasting. Omegas smelled like fruits, and sometimes like other parts of fruiting plants, but were usually sweet.
Usually, not exclusively. Omegas could be crisp, fresh, or even a little savory. Senku wasn’t even that much of an outlier. One of the omegas on the school’s baseball team smelled mostly like those sweet American canned beans that get served with hot dogs. Chili peppers were a fruit; Senku was an omega. He did not smell at all like burning oil to anyone who knew what burning oil smelled like, and he hated both valentines and white day with a passion. He wasn’t omegasexual and it was annoying that other omegas tried to flirt with him so often, although if they were the intersex type like Senku, he might fool around with another male omega as an experiment. He was very glad he wasn’t the cloaca type. He knew the biology and how the fluids flushed and cleaned out the dude’s insides, but that meant constantly cleaning up that flushed mess during preheat and that kind of stink wasn’t for him. He also couldn’t quite be properly clinical about the whole ‘pause the digestive system so you don’t have to poop’ thing; the idea of fasting during pre-heat bothered him on a deep, instinctual level. Maybe that’s just because he wasn’t that type of guy and comfortable with his body as it was. He became absolutely ravenous those last couple days of preheat, storing up energy for the ordeal.
An alpha of either gender or a beta guy would be acceptable if he was interested in dating at all. The tentacle-type alpha girls so popular in porn were kind of frightening, but not off the table. The intersex ones weren’t too different from his own body, just with the emphasis switched around. Maybe that was the main issue: Senku didn’t really like other people touching him. Touch aversion was more common among autistics, he knew the statistics and so did his closest friends.
He was also very glad for suppressants that meant he didn’t need to pick a partner for his heats at all. Byakuya had him signed up for a nice clinic even before it was necessary so that even if his father wasn’t in the country Senku would have a safe guarded place to nest and hang out with other omegas during his preheat and then he’d be locked into a private room for the duration of his actual heat. He had everything he needed inside. A small shower and toilet, a kitchenette with a tiny toaster oven, salty snacks and juices, toys to satisfy his hormones, even super fast wifi for when he was between heat spikes. It let him be completely independent, with only a bit of cramping and an extra day or so of heat-induced agoraphobia as a side effect of the suppressants.
Science was wonderful. His core passion was aerospace engineering, but all of it was exciting and Senku wanted to study as much of it as he could. He was grateful for the medicine that forced him to only have easy false heats instead of breeding ones even after he was old enough he should have started having real ones, the computer programming that let him study or play a video game between waves, the ebook reader that didn’t absorb scents so his favorite books weren’t ruined by soaking up heat pheromones, the electronic lock that wouldn’t let anyone with an elevated temperature in even if they were authorized, the air scrubbers in the clinic so he could only smell the things he brought in to nest with… a true paradise of modern convenience.
After thirty-seven hundred years of being encased in stone, all of that was gone. He even missed the annoying classmates that gave him chocolate on Valentines day. Not much, but he was hungry.
It turned out that operating on a calorie deficit despite his best efforts for six months and being entirely alone meant he stopped having heats entirely. He had one weak false heat starting fifteen days after he broke free where he’d made the most embarrassing high-pitched omega yips for hours trying to alert someone to his distress. Studies showed that in ideal conditions those yips could be heard three kilometers away, and his instinct-driven climb to the highest place he could get without leaving his nesting range to sound off probably got his calls close to max distance.
He was a little worried he’d go into a full breeding heat after Taiju woke up, but while his best friend was an alpha Senku’s body just wasn’t up to it. He was kind of cranky around Christmas and New Years, but it didn’t amount to anything. Taiju had a rut every other month, but he ran off to hang out at Yuzuriha’s tree during the spikes. Senku brought him food a couple times, careful to approach from downwind and only get close when he was asleep unless Taiju came back to their camp to check on Senku platonically and patrol the area like the protective idiot he was. What was a pacifist alpha going to do if he found something while patrolling? Big oak, smelling like wood and sawdust and getting rut pheromones everywhere that were trying to trick Senku’s lizard brain into feeling safe.
He didn’t think Taiju would ask him for anything gross if he noticed Senku showing up with food at the tree during one of Taiju’s hormone spikes, but there were some questions he didn’t really want an answer to. Not many, but that one. That one could go unasked forever.
Thinking back, he didn’t think Tsukasa realized Senku was an Omega at all. He’d only been awake a week before he went mask-off as a murderer. Eventually, the food Tsukasa provided would have kicked Senku out of starvation mode, and maybe when that hint of cucumber scent came back Tsukasa would realize Senku wasn’t a fiery alpha and potential rival but a strongly scented omega he could seduce, but he just hadn’t had the chance. On some instinctual level Tsukasa probably noticed. Senku hadn’t been able to help his reaction when Tsukasa promised to protect and provide for him. Belly full of high-calorie fatty meat directly handed to him by Tsukasa, which Senku had seasoned and cooked after a day crafting things at their camp like a stereotypical domestic omega, Senku had bent into a bow that exposed his neck in appreciation without really thinking about it. He had even spent the evening nesting - there was no other word for the compulsive tidying up even if he didn’t really have anything to nest with - and even accepted it when Tsukasa scented him. At the time, he thought Tsukasa was following Taiju’s lead in clasping Senku’s arm and then rubbing his other wrist down Senku’s shoulder and along his side or front, a marking gesture of a weaker and protected member of the pack.
Between alphas, it signified total dominance over the other. Peer alphas clasped each other’s forearms to touch the concentrated patch of scent glands on their wrists to the other’s clothing or skin instead, leaving scent on the forearm instead of the upper arm and torso. Senku’s acceptance likely contributed to how confused Tsukasa was when Senku fought back against his ideology with a crossbow. Taiju told Yuzuriha it was weird how violent it got so quickly. He’d interpreted the scent-marking the same way Senku did: Tsukasa claiming the only available omega as part of his pack, which Senku was fine with short term. Lack of options for everyone involved, really, and he couldn’t take Taiju from Yuzuriha even as a one-off. She’d understand if it happened, but he didn’t want to put his beta friend through the stress of wondering if her best friend would take her boyfriend for the sake of pack dynamics. If Tsukasa had instead been asserting dominance, well, that did explain how easy the initial escape was and why Tsukasa was so willing to kill him after he caught up.
Now Senku had to wonder if how he treated his secondary gender as something that should go without saying and bad habit of ignoring things unrelated to what he was focused on had caused a similar interpersonal blunder. The awkward moment when Kohaku said she appreciated him and he shut her down when they first met could have only been because she meant it platonically, but also could have been a ‘ew, I don’t swing that way’ kind of reaction on her end. They’d been friendly since, no alpha/omega drama. Same for Chrome. He liked that, he liked being treated as a person first and wasn’t stable enough to waste time on romance, so why poke at it? He hadn’t mentioned being omega at all because it was self-evident and irrelevant to the survival situation he was in, but even if he thought it was obvious he did often need to announce it before people fully realized what he was.
On the other hand, when Chrome had his rut in July he’d made sure Senku knew it was coming so he could evacuate. Kinro and Ginro helped Senku make a safe place to hide while Chrome took care of himself, a lean-to leather tent propped against a sturdy bit of cliff with enough of an overhang to be sheltering but not a landslide risk. Chrome was clever, but that could have just been common courtesy. If Kinro and Ginro did it so Chrome didn’t have to do more work on top of preparing for his rut instead of so Chrome wouldn’t know where they hid Senku, maybe they really didn’t realize what Senku’s sex and gender identity was.