It was nighttime, in a lit room that looked like it belonged to a high-class or wealthy person of this age.
A small boy with blue eyes and black hair, wearing a shirt and trousers, stood near a large bookshelf filled with countless books, reading a huge tome with deep concentration.
Then suddenly, the door to his balcony opened, and a demon, Upper Moon Rank Three Akaza, kneeled before him, bowing respectfully.
"I am here to report to you, Lord Muzan," Akaza said.
The little boy turned to look. Suddenly, his blue eyes turned crimson. He spoke in a voice that didn't sound like a child's at all. "Have you found what I asked for?"
Akaza, still kneeling, replied, "I looked into it but came up empty, having no way to even confirm its existence… I was unable to locate the Blue Spider Lily."
The boy asked coldly, "And?"
Akaza continued, "I will continue to do my utmost to live up to your expectations, Lord Muzan. But… I couldn't eliminate the Demon Slayers, because I fought a powerful person who stopped me."
The little boy walked closer to him in silent anger.
"You failed, Akaza," he said.
He raised a single finger. An invisible shockwave struck Akaza instantly.
Akaza's body trembled as veins bulged under his skin. It was as if his insides were being torn apart.
The boy's voice sharpened. "You couldn't even defeat someone who isn't a swordsman. What I want is for the Demon Slayer corps to be annihilated."
He tore a page from the book he was holding.
"Pummel the life out of every last one of them and never let another one enter my sight again," he said, tearing another page.
"It's hardly a complicated task, and yet…" he ripped the book completely in half, "I am still waiting for it to happen. What do you have to say for that?"
He stepped closer. "You couldn't even best someone unarmed and not even a Demon Slayer at that. Why did you fail to kill them, after I sent you there because you happened to be nearby? Akaza… Akaza… Akaza… Akaza!"
Each repetition sent waves of invisible agony through Akaza's body.
Blood began to leak from his eyes, nose, and ears, but he said nothing, enduring the punishment in silence.
Finally, the boy turned away.
"How far you've fallen, Upper Three," he said coldly, walking back toward the bookshelf. "Dismissed."
Akaza disappeared.
The boy, Muzan, paused, wondering who that new person was who had matched Akaza in strength.
"But it didn't matter". He thought, "Since that boy couldn't defeat Akaza and was injured so badly, he can't be that dangerous".
"It doesn't matter who appears," Muzan muttered, taking another book. "I will achieve what I desire."
Back in the Butterfly Mansion, Akira lay on his bed in the same room he and Erika used.
Right now, he was completely wrapped in bandages like a mummy, hands and legs in casts suspended in the air.
The recent fight had caused some real damage to him, especially Akaza's final attack, which broke many of his bones.
Except for his hands, which he used for the Falcon Punch, the rest of the wounds weren't serious thanks to his healing factor, at least it should have been.
But he'd forgotten something; his healing worked through his evolved stem cells. It had healed him, yes, but without realigning the broken bones.
He only noticed afterward, so they had to break them again and put him in casts to heal correctly. It was an excruciating process, but by now, Akira was used to it.
He muttered, "I wonder if I should evolve my periosteum next… so it can rearrange my bones automatically."
Suddenly, he heard Selena's voice telepathically in his mind. "Not a bad idea, but brother, before you permanently choose to evolve parts of your body, think and ask me first, okay? We don't want you to evolve in a weird way, right?"
Akira sighed. "Yeah, don't worry. I won't do it without discussing it."
Selena's tone shifted. "By the way, brother…"
"What is it?" Akira asked.
"Why did you not kill Akaza?" she asked.
Akira replied, "What should I do? He's stronger than me, and I didn't have any techniques developed yet."
Selena's voice turned sharp. "I know. But if you wanted to, you could've killed him easily. You didn't even use the poison you created, not to mention your device, Eclipse. Why didn't you?"
Akira replied, "I didn't poison him because I wanted to fight him fair. I told you that, right?"
"Then what about your device?" Selena asked.
"Well, you see, there are two reasons," Akira said. "Number one: I don't want Muzan to take me that seriously yet. Things would go way out of order if I revealed it".
"That makes sense, but what about number two?" She asked.
Akira continued, "Number two: I'm trying to unlock Transparent World. I thought if I fought him seriously, I might awaken it, but I didn't manage to do it."
Selena said, "So since you couldn't do it now, you let him go… so you could try again?"
"Yep," Akira replied casually.
"But you just let a demon go", Selena said. "When you could have killed him. He might kill and eat a lot of humans, you know".
Akira shrugged, "What should I do about it? I can't save everyone, right? Besides, that's not the only reason. Muzan discovering my full power is the first reason. The Transparent World was just an extra benefit."
"But people of this world will suffer more", Selena pressed.
"If we fail here, people might suffer even more," Akira said flatly.
"But innocent people will suffer", Selena replied.
"As I said, I can't save everyone. Wait…why are you so into this? I don't think you cared that much about innocent people." Akira asked in confusion.
"Yeah, I don't, but… Erika-chan might get mad if she found out, right?" Selena said in a teasing tone.
"That's true," Akira knew. Erika is a kind-hearted person, unlike him, and he knows she will be mad if she finds out. "But that's only if she knows, right…"
Suddenly, a voice from the door interrupted. "Oh, you're going to hide it from me, then?"
"Exactly. It's better if you…" Akira froze mid-sentence as he recognized the voice.
He turned his head toward the door.
Erika stood there holding a tray of food, smiling at him, though her eyes were cold.
"So you're planning to hide it from me?" she asked.
Akira sighed internally. "Yep, she set me up, again… okay, how do I handle this?"
Erika stepped closer. "So what do you mean by letting him go on purpose?"
"Wait, Erika, I can explain…"
"Sure," she said, smiling sweetly. "Explain."
But Akira had no idea how to. All he could do was scream in his mind, "Sister! Mark my words, vengeance will be mine!"
There was no reply; Selena had clearly cut him off on purpose, again.
"I… I… well…" he started, then stopped. After thinking, he decided to be honest.
He took a deep breath and explained. "I didn't completely let him go for no reason. I can't let Muzan realize too much about my power, and I needed him alive so I could learn new techniques. But I need to fight him seriously again to do it. I don't think I can find any other demon I can use for that. He's the perfect difficulty to achieve it. The demons below him couldn't push me like this, and the ones above are too strong. I can't risk them hurting me or the others."
Erika stayed silent for a while, then said softly, "I see."
She sat down on the chair next to his bed.
"Thank you for telling me the truth," she said.
"Are you mad?" Akira asked cautiously.
"I'm mad," Erika said. "But I also understand your reasoning", She sighed. "Now, Open your mouth."
"Huh?" Akira blinked.
"You can't eat food on your own, right? Open your mouth." She said holding the bowl of those glowing energy jelly thing.
Akira felt like he couldn't refuse her, so he did. But instead of the gentle feeding he expected, Erika started shoving jelly into his mouth without saying a word.
Akira had to use his superhuman reflexes just to eat as fast as she was feeding him.
When she finally finished shoving all the food in, she poured water into his mouth.
Akira was more exhausted than hungry now, well, at least he is no longer hungry.
Erika stood up slowly, turning toward the door. "Wait, Erika, about these casts," Akira said. "I think all my bones have already healed. Can you ask them to remove them?"
Erika turned back, smiling coldly. "No. You need to stay in there for a few more days."
Then she left, without showing any mercy.
"NOOOOOO!!!" Akira screamed from inside his cocoon of bandages.
Outside the room, Erika sighed. "Selena, did you do that for me or for him?"
Selena's telepathic giggle echoed in her head. "A bit of both, I guess".
"Why?" Erika asked.
"Didn't I tell you, Erika-chan? Being with my brother isn't easy. He's not a bad guy, but he's not a hero of justice either".
Erika sighed again. "Then what do you mean by both of us?"
"Because I want my brother to become more human. I want him to understand emotions he previously lacked. He's starting to have his own feelings, especially for you, whether he realizes it or not. That's why he listens to you and cares about you. And he won't get mad, no matter how hard you punish him". Selena explained.
Actually, the real reason he wasn't getting mad at her no matter how much she pushes him around is because was because he was felling these were emotions, for the first time in both his lives, and his only reference for such things came from anime, but Selena didn't reveal that.
She continued. "So, I want to use that to make him a better person, someone more human. And at the same time, I'm using him to help you get used to the cruel reality of the world".
"So you're using both of us," Erika said quietly.
"Don't misunderstand, Erika-chan. Both of you are dear to me. But I was born to protect all of reality. I'll use everything I can to do that, but I won't let either of you suffer, so don't worry." Selena said, sure she might be calculative but she still loves them.
"...Whatever," Erika said softly. "Besides, I can't even kill a demon, so I have no right to criticize him. I need to grow too."
She spoke as if making a quiet decision.
Selena asked, "By the way, Erika-chan, are you really going to leave him in that cast for a few more days?"
Erika sighed. "I'll release him in the evening."
Yep. In the end, she didn't really have the heart to make him suffer that much.
A few days passed like that, as they rested in the Butterfly Mansion along with Tanjiro's group.
Erika did release Akira that evening, under his thankful gaze. He had mostly healed, except for his hands, which took more time.
Erika continued to feed him, this time normally. She was still a bit mad at him, but when he used his secret technique, the "Puppy Eyes Persuasion," she couldn't stay mad for long. Not that she was truly angry to begin with.
Time passed peacefully. Akira and Erika finally met Nezuko, they hadn't before since she'd been asleep most of the time.
Erika quickly grew close to her, treating Nezuko like a little sister.
As for Akira… well, he was Akira. He wouldn't try to get close to someone new unless he had to.
Days passed as he healed.
Then one day, Insect Hashira Shinobu visited them.
Akira looked up from his bed. "It's rare for you to visit us, Shinobu-san. Is there an important situation?"
Shinobu sighed. "I visit twice daily to check your condition, though."
Erika elbowed him lightly. "Don't worry, Shinobu-san, he's just joking. So, what is it you wanted to talk about?"
Shinobu folded her arms. "Akira-san, you said you wanted to learn more about demon physiology, right?"
Akira nodded. "Yes."
"That's why we're going on a mission," Shinobu said.
"A mission?" Akira asked. "To hunt a particular demon?"
Shinobu shook her head. "No. She isn't dangerous, she's friendly. We're going to meet a demon named Tamayo. She's a demon doctor. We'll be doing some collaborative research with her."
Akira already knew who she was, but Erika asked curiously, "A friendly demon? Like Nezuko?"
"It's complicated," Shinobu said. "I'll fill you in later."
"So, how are you going to find her?" Akira asked.
"Don't worry, we know her location," Shinobu replied.
"And when are we going?" he asked.
"Tonight," Shinobu said simply.
"Wait, seriously?" Akira blinked, isn't it way too soon.
"Yes. There's no need to waste time," she replied. "So please get ready before tonight."
Akira frowned. "Wait, how are we traveling?"
His bike was sent to the swordsmith's village for repairs, and Erika couldn't fly while carrying two people.
"Simple," Shinobu said, smiling. "We're walking. So, get ready soon."
Then she left.
Akira froze, eyes wide. "Walking!?"
He silently screamed telepathically so only Erika and Selena could hear him, "NOOOOOOO!"
When asked why he did it, he replied flatly, "Because I didn't want to disturb anyone else."
He was punished a little for that, of course, just another day in the life of Akira before his next journey began.