Chapter Text
Chapter One
The Demon Who Remembered Humanity
Harry Potter died after a long and peaceful life.
Then he opened his eyes again beneath a foreign sky.
He was cold.
Hungry.
Alone.
The world around him was unfamiliar—old Japan, centuries before anything he remembered. Forests stretched endlessly in every direction. Mountains rose like dark teeth against the moon.
Then the pain began.
His body twisted.
His senses sharpened.
His heartbeat faded.
And when the agony finally stopped, Harry understood the truth.
He had become a demon.
Not by choice.
Not by desire.
But by fate.
The hunger came next.
It clawed at him.
Whispered to him.
Urged him toward warmth, blood, and human flesh.
Harry hated it.
So he ran.
Deep into the mountains, far from villages, roads, and sleeping homes. He survived on fruit, medicinal herbs, and, when mercy allowed, blood freely given by injured travelers and sympathetic physicians who chose to help the strange creature that had saved them.
It was never enough.
But it was enough to keep him from killing.
Years passed.
Rumors spread.
A monster in the mountains.
A demon who carried lost children home.
A shadow that killed other demons, then vanished before dawn.
A creature that protected humans instead of devouring them.
Eventually, the Demon Slayer Corps came looking.
The patrol found him during a storm.
Their captain attacked without hesitation.
Harry did not strike back.
Instead, he turned.
A child had been hiding nearby.
Another demon lunged from the trees.
Harry moved faster than the Slayers could follow.
He shielded the child with his own body, drew his blade, and destroyed the attacking demon in a single emerald flash.
Then he stepped away.
Slowly.
Carefully.
He placed his sword on the ground and knelt before them.
“I won’t ask you to trust me,” Harry said quietly.
“I’ll only ask that you watch.”
So they watched.
They bound him.
Questioned him.
Tested him.
And finally brought him before the first Master of the Demon Slayer Corps.
The Master studied him in silence.
“A demon,” he said softly, “who bows before humans.”
Harry lowered his head.
“I remember what it means to be one.”
The Master’s expression softened.
“Then perhaps you can remind the rest of us.”
From that day forward, Harry Potter became the only demon officially recognized by the Corps.
There was no title for what he was.
So the Corps created one.
Guardian Hashira.
Not because of a breathing style.
Not because of rank alone.
But because his duty was protection.
His Nichirin blade burned a deep emerald, changed forever by the strange union of human resolve and demonic nature.
Muzan Kibutsuji noticed.
Of course he did.
“You refuse your nature,” Muzan said when they first stood across from one another.
Harry’s eyes were calm.
“No.”
His hand tightened around his emerald blade.
“I refuse yours.”
Muzan sent demons after him for centuries.
None returned with useful answers.
And so time passed.
Hashira rose.
Hashira fell.
Masters aged.
Children became warriors.
Warriors became legends.
Legends became names carved into stone.
Harry remained.
At every funeral, he stood at the back.
Silent.
Still.
When prayers ended, he bowed once.
Then returned to training the living.
His first true student eventually surpassed him in swordsmanship.
When it happened, the young Slayer trembled, expecting disappointment.
Harry only smiled.
“Good.”
The student stared.
“You are not angry?”
“No.”
Harry sheathed his sword.
“You should surpass me.”
That lesson became legend.
Harry never trained successors to imitate him.
He trained them to exceed him.
Centuries later, Gyomei Himejima came to headquarters.
Most people feared Harry at first.
Gyomei simply bowed.
“I hear your footsteps,” Gyomei said.
“They carry no hatred.”
Harry returned the bow.
“You hear more than most.”
From then on, the two often sat beneath the wisteria trees in comfortable silence.
Sanemi Shinazugawa was different.
Sanemi hated demons.
Therefore, he hated Harry.
Their first meeting nearly became a duel.
“You’re still a monster,” Sanemi snarled.
Harry nodded.
“I know.”
Sanemi paused.
“That’s it?”
“I’m not asking you to forgive what demons have done,” Harry replied. “I’m asking you to judge me by what I choose.”
Sanemi did not accept that answer for years.
But Harry never demanded that he should.
Shinobu Kocho approached him as a medical mystery.
She studied him.
Questioned him.
Tested samples.
Observed his discipline.
At last, she said, “I expected to find a weakness.”
Harry smiled faintly.
“And?”
Shinobu looked at him for a long moment.
“I found discipline.”
Giyu Tomioka understood him almost immediately.
Neither man enjoyed unnecessary conversation.
Their training sessions often passed without a single word.
The other Slayers eventually stopped interrupting.
Kyojuro Rengoku, however, considered silence a challenge.
“GOOD MORNING, HARRY!”
Harry looked up from sharpening his sword.
“…Good morning.”
Rengoku grinned.
“I SHALL MAKE YOU SMILE TODAY!”
It took six months.
When Harry finally laughed at one of Rengoku’s stories, the Flame Hashira celebrated as though he had defeated an Upper Rank.
Mitsuri Kanroji never believed Harry was frightening.
She brought him homemade meals constantly.
Then remembered.
“Oh!”
Her cheeks flushed.
“Right. You can’t eat this.”
Harry accepted her kindness anyway.
Eventually, she brought flowers instead.
Harry pressed each one carefully into books.
Muichiro Tokito once asked, “How old are you?”
Harry considered it.
“I stopped counting after two hundred.”
Muichiro nodded.
“That sounds tiring.”
Harry looked toward the mountains.
“It is.”
Obanai Iguro watched him for years.
Carefully.
Suspiciously.
Then one evening, beneath falling wisteria petals, Obanai said, “I have never seen you lose control.”
Harry’s answer was quiet.
“I have wanted to.”
Obanai looked at him.
“Many times.”
Harry sheathed his blade.
“That is why I trained.”
Obanai never questioned his loyalty again.
Then came Tanjiro Kamado.
The boy arrived at headquarters and froze at the sight of a demon wearing the Corps uniform.
Harry noticed immediately.
“I know,” he said gently. “It’s unusual.”
Tanjiro bowed.
“My sister is also a demon.”
Harry’s expression softened.
“I know.”
He looked past him, toward the small figure nearby.
“I’ve been waiting to meet both of you.”
Nezuko Kamado approached without fear.
She sniffed Harry once.
Then patted his arm.
Harry chuckled.
“I’ll take that as approval.”
Tanjiro exhaled shakily.
“If Nezuko trusts someone,” he said, “I usually do too.”
Zenitsu Agatsuma screamed the first time he saw Harry.
“A DEMON HASHIRA?!”
Harry calmly offered him tea.
Zenitsu accepted it with trembling hands while continuing to panic.
Months later, Harry became one of the few people whose quiet presence could calm Zenitsu before a difficult mission.
Inosuke Hashibira reacted exactly as expected.
“I’M GONNA BEAT THE DEMON!”
Harry defeated him in approximately four seconds.
Without drawing his sword.
Inosuke lay flat on the ground.
Silent.
Then he lifted his head.
“…Teach me.”
Harry smiled.
“I was hoping you’d ask.”
Kagaya Ubuyashiki trusted Harry completely.
To new recruits, he often said, “If Harry gives you advice, listen.”
He would smile gently.
“Not because he is always right.”
“But because every lesson he teaches has been paid for with centuries of experience.”
By the time Tanjiro’s generation joined the Corps, Harry was no longer merely a Hashira.
He was living history.
Every training hall carried stories about him.
Every Hashira had learned from him.
Every Master had trusted him.
New recruits expected terror.
They expected claws.
Fangs.
A monster wearing a human face.
Instead, they found a quiet man beneath a wisteria tree, patiently sharpening an emerald Nichirin blade while sparrows perched on his shoulders.
One recruit whispered, “Is… is that really the demon?”
An older Slayer smiled.
“Yes.”
The recruit swallowed.
“And we’re safe?”
The older Slayer looked toward Harry, who had carried the Corps through centuries of grief and war.
“The safest place in headquarters,” he said, “is wherever he’s standing.”
Harry looked up.
His eyes were tired.
Kind.
Ancient.
“Welcome,” he said softly.
“I hope your stay here is a long one.”
For a moment, the recruits forgot he was a demon.
They saw only a teacher.
A guardian.
A man who had lost his humanity in body, but never in spirit.
And beneath the wisteria trees, Harry Potter continued doing what he had done for centuries.
Protecting the next generation.
So they might become stronger than the last.