Kibutsuji Muzan had very little interest in raising children.
Turning a child into a demon because of pity, sympathy, a flicker of shared experience, or any other reason was one thing. Keeping that child by his side was another. Personally teaching and raising that child was something else entirely.
He had no experience with children, and he had no desire to fill in that gap through firsthand practice.
So the only reason a child like Rinko was allowed to stay by his side at all was because the boy happened to possess certain… pleasing qualities.
Yes. Pleasing.
His dislike of the word "child" came from experience. Noisy, loud, overly energetic, far too lively. There were plenty of negative adjectives that could be used to describe a typical child.
The good news was that Rinko had none of those traits. He did not yell. He did not throw tantrums. He did not shout. He did not make demands.
After becoming a demon, his frail human body was no longer a limitation. Even if he still looked weak, there was no child whose vitality could compare to Rinko's now. Even if his neck were twisted, his limbs broken, his body cut into three pieces, the instant the attack stopped, Rinko's body would regenerate. Having absorbed more blood than most, he possessed an impressive self-healing ability.
Which meant Rinko could, very easily, cause a certain amount of harmless trouble for Muzan.
But he did not.
Rinko had his own mind, his own way of thinking, yet he never once tried to cause trouble on purpose. Muzan believed this was the residue of a human survival instinct. It was neither good nor bad in itself, but it made Rinko especially likable.
It was undeniable that such a child was a very suitable living thing to keep at his side to pass the time.
Teaching him to read and write, to recognize things, to understand the world around him was a leisurely activity quite suited for killing hours. Rinko, for his part, showed tremendous interest. He loved learning new things and exploring the unknown, as long as he had permission.
But at the end of the day, Rinko was still a child. Even though he was more sensible than most, he naturally produced some problems that Muzan had never anticipated and did not quite understand.
They were small problems, amusing ones. Or rather, Muzan found them amusing precisely because it had never once occurred to him that such problems could exist.
Maybe it was because Rinko was still young, and that half-developed brain could not grasp how different his body was now from before. Or perhaps it was simply the lingering regret and hunger of his human flesh. Whatever the cause, the result was this.
Rinko would, without thinking, put human food in his mouth, as if he were still human.
It was only after the food passed his tongue, slid down his throat, and fell into his stomach that he would suddenly realize he had eaten the wrong thing and then vomit every bit of it back up.
When Muzan questioned him, he discovered that Rinko's sense of smell, taste, and rational judgment did not quite match up. His nose still detected scents the way a human's would. Food smelled like food. It smelled delicious. But at the same time, human blood, with its metallic tang, also registered as something edible. His tongue, on the other hand, could not taste much of anything. Only after something entered his stomach did that organ start to complain about whether what had gone in was "correct" or not.
It sounded troublesome, but in truth it was not.
A demon's body could withstand this kind of harmless abuse. That brand-new stomach would not be ruined just because he swallowed the wrong thing. Even if he did eat something he should not, it would no longer leave him collapsed on the floor and unable to move. At most, he simply felt uncomfortable while he was throwing it back up.
So Muzan had not intended to force Rinko to change. It was just that Rinko's curiosity about eating far exceeded Muzan's expectations.
Even knowing that many things were not meant for him, Rinko kept trying anyway.
Demons could not truly eat human food. The taste was disgusting. Even if they forced it down, they could not draw any energy from it. Yet this small demon boy endured all that discomfort, trying again and again, until he finally found something he could hold in his mouth without immediately having to spit it out.
It was completely unreasonable. Utterly so.
Muzan had watched with his own eyes as Rinko put the candy into his mouth, then simply held it there without spitting it out. Even when they returned home, the candy was still there.
While Muzan tried to figure out why, he also wondered whether he ought to reward Rinko's bold experiments or lecture him for such a pointless act.
Rinko had received more of his blood than other demons. By all logic, that meant he should be stronger than the rest. Just as his regeneration surpassed that of ordinary demons, his combat strength should also have been superior.
Experience told Muzan this was how things worked.
Reality did not agree.
Judging by appearance, aura, or actual performance, none of it matched that expectation. Rinko had the regeneration a powerful demon should have, but not the corresponding combat ability. He had no interest in fighting. He was not good at it. In fact, he did not know how to fight at all. He did not know how to attack or how to kill. Even though his current strength should have allowed him to easily tear an adult to pieces, when he actually encountered adults, there was a deep-seated fear.
Aside from treating Muzan with a relatively calm demeanor, Rinko, when outside, would run back to hide behind Muzan whenever an adult man approached him.
He was no different from a human child. If anything, compared to human children, Rinko seemed even more fragile.
If there was one thing Rinko was good at that matched his abilities, it was hiding. He was excellent at disappearing. Whether it was a deliberate game of hide and seek or an accidental case of curling up somewhere and falling asleep, finding Rinko always turned into a chore.
Once, Rinko had fallen asleep inside a closet. Muzan, despite using his blood's power to sense demons, had not realized that there was a sleeping child lying in the cupboard behind him the whole time.
Taking demon individuality into account, Muzan decided that this was Rinko's talent.
As a human, Rinko had wanted nothing more than to run away from that painful home, to be invisible, unnoticed. So after becoming a demon, the thing he excelled at was still erasing his own presence, avoiding recognition.
Rinko's lack of combat talent sounded like a problem. But Muzan was not short on demons who could fight. One more or one less made no real difference. Besides, given Rinko's height and weight, even if he learned to fight, his combat power might not compare to other demons anyway. The issue negated itself almost as soon as it arose.
And in Muzan's view, if his guess was correct, hiding would not be Rinko's only skill. Given time, Rinko might develop other related abilities. As for what direction those powers might grow in, that would simply be a surprise waiting to reveal itself. Muzan decided to leave that answer to the future.
Oh, and as for the candy, Muzan only discovered the truth three months after Rinko first managed to eat it.
Rinko was still a demon. He still could not truly consume human food. It was just that when candy dissolved on his tongue, the melted sugar slid gently down his throat into his stomach. Compared to chunks of meat, sticky rice, or other more substantial foods, the presence of that sweetness was too subtle. For a while, his stomach simply did not notice that something "forbidden" had gone in. After he ate several more, and they gradually accumulated, his stomach finally realized what was happening. At that point, of course, it rejected them. What needed to be thrown up was thrown up.
Only then did Muzan finally, belatedly, decide to properly educate him.
"Demons cannot eat human food."
"Then what am I supposed to eat?"
It felt like the story had come full circle.
Muzan had once taught Rinya, the human boy, how to live.
Now he had to teach Rinko, the demon child, how a demon was meant to live.