Lady Amane slid the door closed with quiet grace, her daughters bowing in perfect unison as they followed her out.
For a moment, the Hashira meeting room was peaceful, calm, still, almost sacred.
Then.
BOOOOOOOOM!!!
A deafening explosion shook the far end of the hall.
Rengoku stood trembling, no, vibrating, like a festival lantern seconds away from bursting. His eyes shone brighter than ever, and his fiery hair swayed in waves as if buffeted by an invisible storm of pure energy.
"AAAAAAAAAHAHAHAHAHA!! I FEEL AMAZING!! MY BLOOD IS BOILING!!! MY MUSCLES ARE SINGING!!! MY HEART IS A SUN!!!"
Every Hashira flinched.
Mitsuri gasped.
Giyu blinked, expression unchanged.
Sanemi swore under his breath.
Akira glanced at his clipboard, completely unfazed, and muttered, "…Ah, that's the side effect. Side effect number three of Vitality Boost Type 6: excessive physical output. I forgot about that."
Erika snapped her head toward him. "You what?"
Shinobu's voice rose sharply. "You forgot?!"
Rengoku suddenly sprinted, blasting across the courtyard so fast he blurred into a streak of red and gold.
WHOOSH!!
Tengen pointed dramatically. "OI! HE JUST BROKE THE SOUND BARRIER. FLASHYYYY!"
Obanai's eye twitched. "Is he… bouncing off the walls?"
Shinobu called out, panic creeping into her voice, "Akira-kun, what exactly is happening?!"
"It's nothing much," Akira replied casually. "The serum gave him so much extra energy he can't stop moving."
THUD. THUD. THUD.
Every few seconds, Rengoku ricocheted off another wall like the world's happiest ping-pong ball.
Mitsuri clasped her hands, nearly in tears. "Rengoku-saaaan! Slow doooown!"
"I CANNOT!" Rengoku roared from somewhere above them. "MY BODY YEARNS FOR MOVEMENT! LET ME RUN UNTIL THE SUN RISES!!"
Sanemi pointed upward, horrified. "WHY THE HELL IS HE CLIMBING THE WALL LIKE A DAMN SPIDER?!"
Giyu blinked once. "…He's in good spirits."
Sanemi whipped around. "HOW IS THIS 'GOOD SPIRITS'?! HE'S GOING TO DIE!"
Akira raised a hand in mild reassurance. "Relax. It burns itself out in about… two hours."
Everyone froze.
"…Two hours?" Shinobu whispered hollowly.
"Give or take," Akira nodded pleasantly with a smile.
CRUNCH!
Rengoku dropped back down into the exact center of the tatami mats, landing in a perfect heroic pose, steam rising from his clothes, his grin majestic and borderline unhinged.
"FRIENDS! HASHIRA! I HAVE JUST RUN AROUND THE ENTIRE ESTATE NINETY-THREE TIMES!"
Erika stared. "When did he do that again?"
Akira shrugged. "Probably while we were talking."
Sanemi sputtered, "WE'VE BEEN TALKING FOR A FEW SECONDS!?"
Before anyone could process what was happening, Rengoku suddenly grabbed Akira's shoulders with blazing enthusiasm.
"HOSHINO AKIRA!!! THIS SERUM IS MARVELOUS!! WHEN CAN I HAVE MORE?!"
Akira shook his head awkwardly. "Nope. I don't want you to be addicted to it. You're already showing signs."
Rengoku blinked… then nodded with dramatic sincerity.
"I UNDERSTAND!! I SHALL TREASURE THIS EXPERIENCE FOREVER!!"
And then he zoomed away again.
WHOOSH!!
Leaving scorch marks on the tatami.
"Akira-san…" Shinobu massaged her temple. "Next time you invent something, please. PLEASE, label the side effects clearly. And what was that about addiction?"
"Oh, that," Akira said lightly. "Right, I forgot about that. But it's not as serious as the others."
"The others? HOW MANY SIDE EFFECTS ARE THERE?!" Sanemi roared.
Akira replied with complete seriousness, "…Yes."
Mitsuri ran after Rengoku, waving her arms. "Rengoku-saaaan! Stop running in circles, please!"
Giyu looked at Akira. "So… the machine and medicine can really awaken the Mark?"
Akira nodded. "Yes. Aside from the hyperactivity, there aren't many side effects."
In the distance, Rengoku's triumphant yell echoed.
"BEHOLD, MY FRIENDS!! I HAVE NOW RUN ONE HUNDRED AND TWENTY-TWO LAPS!! THE FLAME OF YOUTH WILL NEVER DIE!!"
Akira sighed. "Maybe I should start working on Vitality Boost Type 7 before using it on anyone else."
"Please do that," Shinobu said flatly. "How long will it take?"
Akira answered, "Well, it took two days to make this… so maybe one day. Though I'm not sure the next test subject can handle it."
Erika asked, "Did you use it on Tanjiro too?"
Akira shrugged. "Well, he volunteered. I would've used it on myself, but my physiology is already different from humans. Though Tanjiro seems to show immunity to the hyperactivity, maybe because I used it on him too much."
Several Hashira muttered sympathetically.
"Poor Tanjiro…"
"This is tragic…"
"That boy deserves compensation…"
Then.
BOOM!
Rengoku was now jumping from building to building for absolutely no reason.
Akira, looking at him and the deadpan expressions of the other Hashira, said:
"…I'll work on it soon. Don't worry."
That night, Akira was back in his room, exhausted from the day's chaos. He sat on his bed with papers scattered around him, scribbling new formulas for the next serum and refining his research notes on the Blue Spider Lily. Lantern light flickered over the messy table, giving the room a quiet, studious warmth.
The door slid open.
And Erika stepped inside.
"Akira, what are you doing?" she asked, walking over.
Akira didn't look up. "Just some plans for a new serum… and other things."
"Other things?" Erika raised a brow and sat beside him on the bed.
Akira sighed. "I need to prepare for what I'm going to do during Hashira training. And Tamayo will be here soon, so I need to work with her to turn Nezuko back into a human… and also research ways to defeat Muzan. I'll be busy for the next few days."
Erika listened quietly, then asked, "If you're that busy, why did you ask to participate in the Hashira training? I thought you'd focus more on your research."
Akira leaned back slightly, looking tired.
"Well… to be honest, I'm still not very good with martial arts or sword skills. I've managed to beat everyone so far because of my overwhelming strength and talent, but I need real training to grow stronger. And where else am I going to find this many strong people to train with? If I learn to fight multiple opponents properly, I'll feel more confident fighting Muzan too."
"Muzan is really that strong?" Erika asked.
Akira nodded slowly. "He is strong, well, mostly he's nearly impossible to kill. From what I heard, he was once almost killed by the Sun Breathing user called Yoriichi."
"Yoriichi… as in Yoriichi Type Zero?" Erika asked.
Akira nodded again. "It was modeled after him. Anyway, Yoriichi almost defeated Muzan… but Muzan split himself into eighteen hundred pieces to escape. Apparently, Yoriichi managed to cut down fifteen hundred of them, but the remaining three hundred reformed him."
Erika's eyes widened. "If he can do that… how can we even kill him?"
"Don't worry," Akira replied calmly. "Me and others are already working on a poison that can stop him from escaping like that, along with other poisons to weaken him. I think we can beat him. But the real problem now is the remaining Upper Moons."
Erika asked. "Didn't we kill up to Upper Rank Three already? So only Upper Rank Two and One are left. Are they really that strong?"
Akira let out a slow breath. "Upper Rank Two, Doma, uses ice. He's strong. But Upper Rank One, Kokushibo… he's on another level entirely. He might even be stronger than Muzan in a direct fight. We can't underestimate either of them, and there is one more dangerous demon left, too."
Erika swallowed. "… a demon more dangerous than them."
Akira nodded. "Yes, it's Nakime. The demon who controls the Infinity Castle."
"Infinity Castle?" Erika repeated.
"It's an alternate pocket dimension," Akira explained. "Completely controlled by Nakime. It's infinite, with its own rules. She can manipulate everything in it. Honestly… she's the most dangerous one. If we get stuck in there, finding her or killing her would be incredibly difficult."
Erika frowned slightly. "Then how are you going to beat her?"
Akira stared at the ceiling, looking both tired and determined.
"Well… I'll find a way to beat that… somehow."
He lay back fully on the bed, exhaling a long sigh.
After a moment, he lifted his head slightly and looked at Erika. "What about you? What are you going to do these days, Erika?"
She thought for a while before answering.
"I'll stay with you and help you with your work."
Akira blinked. "Really?"
"Well," Erika said, sitting a little straighter, "it would be easier if you had an assistant, right? You have too much work after all. I'll help you with it."
Akira sighed, a bit surprised. "I thought you'd train with Mitsuri… or do something else. You've been avoiding me recently after all."
Erika's face immediately flushed. "I am not avoiding you."
"But," Akira continued, "you didn't talk to me much after we returned from finding the blue spider lily. I thought you suddenly hated me or something."
"Are you an idiot?" Erika snapped, flustered. "I would never hate you. I was just… a bit busy with my own thoughts. Nothing else."
"Really?"
"Yes," Erika answered. "Besides, you were busy too, right? I just didn't want to disturb you too much."
Akira smiled softly. "That's good then."
Erika sighed again, rubbing her forehead. "I didn't even know you cared about things like that. You usually never care what others think of you."
"Well," Akira replied, shrugging lightly, "I don't really care what others think of me. But I'm not without a heart."
Selena's voice echoed telepathically, "Brother, you literally don't have one".
Akira ignored her.
"I mean," he continued to Erika, "we used to play a lot when we were little. And we don't play like that anymore, so… I'm a bit sad."
Erika lowered her gaze. She'd been overthinking so much, hearing that made something loosen in her chest.
"Well… we can't just keep playing like children, you know," she muttered.
"I don't mind, though," Akira said honestly. "I like playing. I haven't played with anyone much after we got separated, so I want to go back to how we used to be if possible."
Erika stared at him.
Akira was unpredictable, sometimes mature, sometimes childish.
Because of the strange, broken life he lived, both past and present, his emotional maturity was scattered and uneven.
He wasn't fully grown-up, and yet not fully childish either.
According to Selena, he was high maintenance, someone who wanted to be both adult and child at the same time. And Erika… didn't really disagree.
She sighed. "Fine. I'm not sure how… but I'll try to be as I used to be. Is that okay?"
Akira immediately sat up, eyes brightening. "Okay! Pinky promise!"
"…You and your ridiculous promises," Erika murmured, but she smiled despite herself. "Fine. Pinky promise."
Their fingers intertwined, sealing the childish vow with surprising warmth.
After finishing their planning for the next few days, the room grew quiet. Erika shifted slightly, then spoke softly.
"Alright… time for my daily blood."
"Sure," Akira said, tilting his neck.
Erika slid closer and wrapped her arms around him in a gentle, familiar embrace. Just as she leaned in, however, she suddenly paused and her expression twisting into something mischievous.
Before Akira could question it, she bent down and bit his ear instead.
"GAH! WHY THE EAR?!" Akira yelped, jerking in pain.
Still clinging to him, Erika said innocently, "Well, I can, right? I don't think there's much difference between a neck and an ear."
"There IS a difference!" Akira protested, but Erika ignored him entirely. She bit his ear again, this time nibbling at it with deliberate teasing.
"Hey! That's not fair!" Akira sputtered. "If you do that, then I'll bite you too!"
Before Erika could escape, Akira retaliated and bit her ear.
Erika's entire face went bright red.
She squeaked, "N-No! Only I can bite you, you can't bite me!"
She tried to wriggle away, flustered and embarrassed, but Akira grabbed her around the waist before she could escape.
"I support true gender equality," he declared.
"Akira…wait…!"
And with that, the two of them toppled over onto the bed, rolling back and forth, trying to bite each other like mischievous children. Their laughter echoed through the room, soft, breathless, nostalgic. For a moment, it truly felt like they had returned to those carefree, like children who had rediscovered the joy of simply being together.
Unaware of the chaos above, the Insect Hashira, Shinobu, lay in her futon directly beneath their room.
The ceiling trembled.
The laughter continued.
Shinobu stared at the ceiling with dead eyes.
"…I swear," she muttered under her breath, "if they don't quiet down, I am going to poison them."
And so the night continued, with Akira and Erika wrestling like overgrown children, and Shinobu plotting murder in polite silence.