Following Kibutsuji Muzan's scent, Yōsuke arrived at a medical clinic. Even from outside, the stench of blood was overwhelming.
He slid open the door. Inside, the clinic was in shambles, blood spattered across the floor and walls.
At the very center knelt a woman who had already turned into a demon. She clutched two lifeless bodies in her arms, wailing in despair.
"Why… why did you deceive me?! I only wanted to live… to watch my children grow! My child… my husband…"
For the first time in nearly two centuries, Yōsuke beheld a demon who still retained her sanity. Driven by curiosity, he refrained from striking her down immediately.
The woman noticed him, sensing the demonic aura around him. Her eyes blazed with hatred as she glared at him.
"It's you! You monsters, you demons — it was all your deception!"
She hurled her fist toward him.
But how could a newborn demon ever match Yōsuke? Her blow landed harmlessly on his shoulder, like striking iron.
Yōsuke seized her arm and asked coldly:
"How long has it been since Kibutsuji Muzan left?"
The woman's voice dripped with loathing.
"So that deceiver's name is… Kibutsu—"
Her words stopped abruptly. An overwhelming dread welled up in her chest. Instinct screamed at her: if she spoke that name aloud, she would die.
"Why… why am I afraid to say it?!" she demanded, eyes burning with confusion and rage.
Yōsuke answered evenly:
"The one who made you into a demon is Kibutsuji Muzan. When he transformed you, he placed a curse upon your blood. Any demon who leaks information about him will be corroded and killed by that curse."
"Then why are you unharmed?!" she demanded.
"Because I am not a demon created by him. Like Muzan, I am an Origin Demon."
The woman froze, staring blankly at Yōsuke's face. Then, in a sudden frenzy, she snatched a knife from the fruit tray on the table and drove it toward his neck.
The blade snapped instantly upon touching his skin, as though striking steel.
Collapsing, the woman fell to her knees in despair.
"Why won't you die?! You accursed demons… why can't you die?!"
"You needn't direct so much hatred at me," Yōsuke said calmly. "If you seek Muzan's death, then our goals align. Tell me your name."
Broken, she murmured:
"…Tamayo."
At the sound of that name, Yōsuke's thoughts scattered. Tamayo? The very same Tamayo who, in the future, would develop the medicine to restore demons back into humans? The Tamayo who would play an irreplaceable role in the final battle against Kibutsuji Muzan? The greatest ally of all?
Crouching down, Yōsuke met her despairing eyes.
"I can remove the curse Muzan placed upon your blood. But the price is this: you must work with me to develop a medicine that can restrain him. For you, it is not a price… it is an opportunity."
Though hatred consumed her, Tamayo had already been deceived once by Muzan. She was cautious.
"If you are a demon as well, why would you want that man dead?"
"Though Muzan and I share the same bloodline, we are not the same. I have always considered myself human. My family was cursed because of Muzan… and because of me. My responsibility is to sever this sin."
Tamayo remained suspicious despite the sincerity in his voice.
"Tamayo," Yōsuke pressed, "you have no other choice. Muzan is searching for a way to overcome the sun. A skilled physician like you will inevitably be found by him. And when that time comes, you will not be able to resist."
Her expression wavered. At last, with bitter reluctance, she said:
"…I'll do it."
Yōsuke smiled faintly in satisfaction. He knew she did not yet trust him. But he believed that, given time, she would come to see his sincerity and stand with him.
He cut open his palm, letting his blood drip into her mouth.
As the blood of a Demon King entered her body, Tamayo felt as though she were dissolving from within. Agonizing pain wracked her, twisting her body.
Yet for the sake of slaying Kibutsuji Muzan, she did not cry out — enduring silently, determined.
Muzan had not infused her with much of his blood. Against the flood of Yōsuke's, Muzan's blood was forced out, expelled from her body.
Minute by minute, the torment continued, until at last Tamayo felt her body lighten, as if freed from shackles that had bound her soul.
When it was done, Yōsuke offered her his hand and pulled her to her feet.
"My name is Ubuyashiki Yōsuke. Welcome, Tamayo… my new comrade."
Yōsuke departed from the clinic. Tamayo remained behind, to bury her husband and child.
When Yōsuke next saw her, she no longer carried the grief of losing her loved ones. What remained was only her hatred of Muzan.
To aid her research, Yōsuke hunted down countless demons, bringing her samples of Muzan's blood. He even drew his own blood for her experiments. Yet progress was slow — painfully slow.
But over decades spent together, Tamayo gradually set aside her wariness. Despite being Muzan's younger brother — another Demon King — Yōsuke had never once eaten a human, instead devoting himself to hunting demons.
At last, Tamayo accepted him as her ally.
After slaying a demon , Yōsuke returned to a rural clinic. Sliding open the door, he found a woman seated within, tending to herbs.
She had long black hair, parted in the middle and tied into a large, low bun secured with a floral hairpin. Her eyes were gentle, misted with a faint rose hue; her lips red against her pale skin. She wore a deep purple kimono adorned with wavelike lavender branches and crimson blossoms, bound at the waist by a beige sash.
"Tamayo, how goes the research?" Yōsuke called the moment he stepped inside.
Decades had passed. All the humans they had once known were long gone, leaving only the immortal Yōsuke and Tamayo to keep each other company.
She was used to his boisterousness by now.
"The same as always. No progress."
Yōsuke handed her a vial of blood he had just taken from a slain demon.
"Try this one. That demon had a fair bit of Muzan's blood in him."
After inspecting it, Tamayo shook her head.
"Still not enough. The blood may be plentiful, but the concentration is too low."
Yōsuke scratched his head in frustration.
"Then maybe you should take more of my blood?"
"Even if I drained you dry, it wouldn't help. You and Muzan may be brothers, but your Demon King genetics are entirely different. Think of it like carnivores and herbivores — both animals, but fundamentally unlike. Any medicine derived from your blood would work only on you."
Her explanation left Yōsuke with a pounding headache. Medicine was a world he did not understand in the slightest.
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