When Chika said those words, the room went utterly silent.
Her tone carried such conviction that no one knew how to respond.
Zenitsu's jaw dropped. "S-So cool…"
Inosuke's eyes sparkled. "That's the spirit, Big Sister Boss!"
Tanjiro clenched his fists, fire burning behind his gentle eyes.
"Muzan… I'll kill you with Sister—no matter what!"
The boy who'd wanted to join the Demon Slayer Corps lowered his head. "Then… I'll just pray for you all in secret."
Everyone nodded, accepting his decision without judgment.
Night had already fallen.
At the children's insistence, the group stayed the night. The house wasn't big enough for everyone to have their own room, so they had to make do.
Tanjiro, Zenitsu, and Inosuke all squeezed into one room.
Chika, carrying the wooden box with Nezuko inside, took another.
Once the door closed, the box creaked open from within.
Nezuko crawled out slowly, her expression pure bliss as she flopped onto the floor with a soft plop, limbs splaying out.
Poor Nezuko—stuck in that cramped box all day, unable to move freely or speak.
Chika smiled faintly at her sister's relief. She really needed to find a safe opportunity for Nezuko to start testing sunlight exposure soon.
"Nezuko, want to train your breathing and sword form for a bit?"
"Mm! Mm!"
At once, Nezuko bounced up, sitting neatly like a model student awaiting instruction.
Her progress in Water Breathing wasn't fast yet—she still hadn't reached the point of manifesting any "effects."
But that was fine. It had only been half a month since she'd started formal training.
For now, her effort mattered more than results. Though, if she ever wanted to use it in real battle, they'd need to speed things up…
In the modest room, Chika guided Nezuko through each step of her breathing practice.
Meanwhile, in the boys' room—things were considerably less peaceful.
"Move over."
"I can't! It's already cramped!"
Inosuke on the left. Zenitsu on the right.
Tanjiro trapped right in the middle.
As the two bickered and jostled, Tanjiro somehow stayed sound asleep, breathing evenly, face serene.
"Man, he sleeps like a rock."
"Wait," Inosuke muttered, narrowing his eyes. "His breathing's… weird."
"'Weird'? What, like snoring?" Zenitsu scoffed.
"No. Can't you feel it?" Inosuke tilted his head closer, studying Tanjiro's calm chest. "His breathing's steady—too steady."
Zenitsu, half annoyed, half curious, sat up to look closer.
A few seconds passed. His face went pale.
"Hold on! Wait a second! That's—That's Total Concentration Breathing! He's using it right now! While asleep!"
Inosuke blinked, then did the dumbest thing imaginable—he covered Tanjiro's mouth and nose with both hands.
One second.
Two seconds.
Three—
Tanjiro's eyes flew open as he shot upright. "What are you doing?!"
"Tell us the truth!" Zenitsu demanded. "You weren't really asleep, were you?!"
"Of course I was," Tanjiro said blankly, rubbing his eyes. When he realized what they were talking about, he chuckled softly.
"Why are you laughing?!"
"Because it's fine, really," he said kindly. "I wasn't pretending. My sister and I have both trained to maintain Total Concentration, Constant even while asleep. That's why we're strong."
"T-Twenty-four hours?!"
Zenitsu and Inosuke froze, eyes wide in disbelief.
They'd seen him sleeping. They knew he wasn't lying.
To keep up constant breathing like that for even ten minutes in battle left them gasping for air.
But twenty-four hours? Even while sleeping?! That was insane.
"Tanjiro… you two are monsters," Zenitsu groaned, sweat dripping down his temple.
Inosuke's eyes gleamed. "So if I learn to breathe like that all day long, I can beat Big Sister Boss, right?!"
"Uh… the gap would definitely shrink," Tanjiro said carefully after a pause.
He didn't have the heart to tell them the full truth—that even after mastering Total Concentration, Constant, he still couldn't beat his sister.
Chika's body and instincts operated on a different level entirely. Every spar ended the same way: Tanjiro on the floor.
Still, motivation was good.
"If you really want to learn, I can teach you," Tanjiro offered warmly. "Though I'm not sure I'm a great teacher."
The two looked at each other. Inosuke agreed instantly. Zenitsu hesitated—then reluctantly nodded.
It didn't take long for regret to kick in.
Forcing their bodies into constant focus left their lungs burning, their muscles screaming in pain. Every breath felt like fire.
"Ow, ow, ow! Tanjiro, how do you even live like this?!" Zenitsu wheezed, sprawled face-down on the futon like a dying worm.
Inosuke wasn't much better off, his usually tough frame trembling with strain.
Tanjiro just smiled gently. "It's normal. You'll get used to it. Everyone starts like this."
Get used to it, he said.
At that moment, both of them realized that sleeping peacefully was a luxury they'd taken for granted.
Morning.
Zenitsu and Inosuke stumbled out of their room, stiff and sore, groaning with every step.
Overtraining had hit them hard.
Meanwhile, Chika emerged with Nezuko's box in hand, looking utterly refreshed.
Despite failing yet again to enter the Transparent World, she didn't mind anymore. Nezuko had made major progress with her breathing, and that alone made her happy.
Outside, the familiar black crow circled above, cawing loudly.
Their next destination had arrived—the last trial before the true death zone of missions.
"Next target—Mount Natagumo."