The moment Muichiro was allowed out of bed, he rushed straight into the mountains to find Kokushibo. When he realized Kokushibo hadn't "relocated" this time, he looked even more delighted.
"Long time no see, Big Brother! Sorry I haven't come to visit you in so long!"
"I heard."
Kokushibo lifted his gaze toward him.
"To be injured by something of that level… how incompetent."
"Eh—you knew?"
Muichiro grinned.
"So you're the cold-on-the-outside, warm-on-the-inside type!"
"…Come at night next time. I will evaluate your strength."
"Whoa! Does that mean you're going to train me? You are, right?"
Muichiro stepped closer and crouched beside him, smiling brightly.
"Then can I know your name now?"
No matter how much he pressed, Kokushibo offered no reply. He simply closed his eyes and resumed sitting cross-legged. This wasn't the first time Muichiro had been treated this way. He didn't feel discouraged. Instead, he began enthusiastically recounting his battle with a member of the Twelve Kizuki.
The more Kokushibo listened, the deeper his frown grew.
"To cast away one's life for others without regard for one's own safety… foolish."
"But doesn't it feel meaningful when people thank you?" Muichiro beamed.
"Hiru always says the world only has value when there are more good people in it. Every time I think about helping Hiru make this world just a little better—and saving so many people—I feel like everything I do has meaning. "
"I want to help even more people in the future. If everyone could be happy, that would be wonderful!"
Kokushibo found the sentiment naïve and offered no reply.
Muichiro continued chatting about trivial things until dusk fell, then hurried off, clearly having snuck out again without permission.
Truly lacking in composure.
Even so, the next time Muichiro came, Kokushibo carefully evaluated his swordsmanship. He pointed out dozens of flaws—both major and minor—with ruthless precision, almost to the point of severity, and forced Muichiro to correct every stance and movement.
Muichiro was more than willing to refine his techniques under Kokushibo's guidance. However, he was far less enthusiastic about Kokushibo's lectures on analyzing a demon's Blood Demon Art.
It wasn't that Kokushibo explained poorly. It was simply that his deliberately slowed cadence, paired with lengthy explanations, felt like pure torture.
Every time Muichiro began to nod off, Kokushibo would strike his head with precise force—enough to hurt, but not injure—then frown and remark coldly,
"No ambition."
That only made Muichiro feel deeply ashamed.
This secret arrangement did not remain hidden for long. Within half a month, Yuichiro noticed something was off.
After secretly following Muichiro and learning the full story, he immediately dragged him away and sternly forbade him from seeking out Kokushibo again.
Muichiro argued passionately in Kokushibo's defense, insisting he wasn't an evil demon, that his swordsmanship was exceptional, and that he was an excellent teacher. He even invited Yuichiro to train alongside him.
Yuichiro was nearly driven to internal injury by Muichiro's naïveté. He stopped arguing and instead kept a close watch on his younger brother, leaving him no chance to sneak out again.
Kokushibo, whose instruction had been abruptly halted, felt no anger. On the contrary, he found Yuichiro's vigilance rather reassuring.
When Muichiro attempted to sneak out once more, Kokushibo personally carried him back home, leaving Muichiro utterly devastated.
However, that action greatly alarmed Yuichiro. After days of silent speculation about Kokushibo's motives, he finally wrote to Yoriichi and Hiru, explaining everything.
They did not respond by letter. Instead, the very next evening, both returned to the river valley one after the other. Only then did Yuichiro finally relax.
Yoriichi sent Tanjiro home to reunite with his family. Then he and Hiru separately listened to the twins' account.
As Muichiro spoke, Hiru's expression grew increasingly peculiar. When Muichiro mentioned bringing sweets and medicine to restore a demon's sense of taste, Hiru's lips twitched.
"You really went to deliver sweets," he said at last. "Did you forget the Demon Slayer Corps' entry regulations?"
"But aren't you helping a demon too?"
Muichiro puffed out his cheeks.
"You can do it, but we can't? Isn't that hypocritical?"
"I have the strength to survive. You don't."
Hiru tapped Muichiro's forehead.
"You place blind trust in a vague feeling, without any means of restraint. Were you a fool in your past life?"
"Fool?"
"It means you're stupid."
Hiru folded his arms.
"Seriously… what is Big Brother thinking? We agreed to deal with Muzan first before going after him. And now this happens again…"
"Perhaps he took a liking to Muichiro's talent," Yoriichi said gently, a faint smile of quiet relief on his face. "After all, our brother has always been a gentle person."
"There you go again…"
Hiru sighed and stood.
"Why not resolve this now? That way, we can focus entirely on Muzan afterward."
Yoriichi's smile dimmed slightly, though he did not argue.
"So that is what you believe… But would that create complications in our plan to slay Muzan? My recent investigation is only just beginning to yield results. If you give me a little more time, I can identify patterns from those eyeballs and trace the demon responsible for that Blood Demon Art."
"Right… Muzan is more important."
Hiru pressed a hand to the corner of his eye.
"The real issue is whether Muzan still maintains a connection with Big Brother. If he senses something has happened to him—whether he withdraws for a time or mobilizes everything at once—it would be troublesome either way."
"…What would happen to Big Big Brother?"
Muichiro, who had been silent, finally spoke.
"Why?"
Hiru looked at him, his expression indifferent.
"With a demon's regenerative nature, what else could 'something happening' mean? Only death."
Yuichiro also looked stunned.
"But aren't you family? You and Yoriichi helped Tanjiro. Why…"
"Yes! Exactly!"
Muichiro's face flushed with anger.
"Isn't Big Big Brother trying to restrain himself? He hasn't harmed anyone. He isn't aggressive. Even if he has some strange quirks, those can be tolerated!
"Why are we suddenly talking about killing him? Why are you harsher on your own family than on anyone else? You care so much about Tanjiro's situation! But when it comes to your own elder brother, you're this cold?"
"It makes no difference who it is."
Hiru's expression remained ice-cold.
"A demon who has eaten humans does not deserve redemption."