A roar of flame burst across the training grounds, shaking the entire Demon Slayer Corps headquarters as if it were boiling over.
Inside Chika's temporary quarters, Inosuke and Zenitsu sat cross-legged on the floor, black cloths tied over their eyes as they tried desperately to maintain Total Concentration Breathing: Constant.
Tanjiro paced around them, his footsteps intentionally loud, trying to break their focus. Sometimes he went further—
A light pat on the head.
A sudden tickle on the soles of their feet.
Inosuke could handle such distractions with brute will, but Zenitsu? Every time Tanjiro touched him, his concentration shattered instantly. He was on the verge of losing his mind.
Tanjiro rubbed the back of his neck awkwardly. "This isn't something I made up…"
He remembered when they had trained under Urokodaki-sensei—the old master's method had been far more brutal. Back then, Urokodaki simply told them: 'Keep your breathing constant. If it breaks, my club will teach you why it shouldn't.'
Just recalling the strength of that wooden mallet made Tanjiro's shoulders flinch.
At that time, Chika, sitting beside him with a serious expression, had raised her hand.
"Urokodaki-sensei, I think that method isn't very practical!"
The old man blinked, puzzled, then gestured for her to continue.
Tanjiro had assumed his sister, like him, was afraid of the beatings. But her next words made him realize—he had completely misunderstood.
"Sensei, maintaining Constant Breathing isn't something achieved overnight. During the day, you can supervise and hit us when we slip, but what about when we sleep? You can't stay awake all night watching us, can you?"
Her tone was firm, logical.
Urokodaki fell silent, lowering his head with a rueful smile. She was right. Unless he had an assistant—or at least one disciple who didn't need to train—he couldn't possibly watch them every night.
Seeing his hesitation, Chika pressed on.
"So instead, let Tanjiro and me supervise each other! We're young—we can take turns staying awake. That way, you can rest, Sensei."
Her reasoning touched the old master deeply. Such thoughtful children…
He finally agreed. From then on, Urokodaki only explained the principles and a few helpful tips; the rest of the Constant Breathing training was left entirely to the siblings themselves.
Now, as Tanjiro watched Zenitsu and Inosuke struggle, he couldn't help but smile at the memory. Compared to what he went through under Chika's supervision, their training was practically gentle.
"Keep it up, everyone! You're both improving so fast—I'm sure you'll master it soon!" he encouraged, clapping his hands.
Though both Zenitsu and Inosuke secretly wanted to quit, neither could bring himself to do it. Each looked at the other and thought, If he's not giving up, how can I?
That silent competition pushed them to keep going.
From morning until now, they hadn't broken concentration once.
Until—
BOOM!!!
A thunderous explosion rattled the room.
Both Zenitsu and Inosuke flinched violently, their breathing shattered. They fell backward with a thud.
"Tanjiro! What was that?!"
Still blinded by their cloths, they called out—but got no response.
Panicked, they fumbled to untie the fabric. It was knotted too tight, and without sight, it took them a full minute to tear it off.
When they finally looked around—Tanjiro was gone.
But the noise outside, that roaring flame, told them everything they needed to know.
They grabbed their Nichirin Blades and dashed toward the commotion.
By then, Tanjiro was already at the training grounds. His sharp nose had caught the scent of burning air and intense emotion even before the explosion.
That smell… Fire God Kagura? Sister's using it?
He frowned, just as the explosion erupted, confirming his suspicion. The direction—it was exactly where Chika had been training.
"Chika—!"
Without hesitation, he sprinted full speed.
By the time Zenitsu and Inosuke reached the courtyard, Tanjiro was already there.
The field was thick with black smoke. In its center stood Chika, her Nichirin Blade glowing red-black in her hand.
Across from her, Rengoku Kyojuro braced his stance, sword raised, still holding his ground.
That last strike—he had blocked it in the nick of time.
Rengoku's breathing was heavy, and his wrist trembled faintly around his blade.
Her power… completely different from before.
Same fire, but the heat—hers burns purer, stronger than mine.
He exhaled slowly, lowering his guard only for a moment before gripping his sword with both hands again. His fighting spirit didn't waver—it flared brighter.
Was he afraid?
Not at all.
Only exhilarated.
Their eyes met. Neither spoke.
They moved.
Both swordsmen lunged forward at once, blades clashing in a furious storm.
The air rippled with each strike, the fiery aftershocks bursting into small explosions along the ground.
Clang—clang—clang!
At such blinding speed, even the onlookers could see only flashes of red and black.
And as their duel intensified, the heat from their blades grew so fierce that both Nichirin Blades began to glow—
A molten crimson light spreading along the steel.
The mark of blades surpassing their limits.