Deep within Mount Momoyama, the clash of blades rang out without end.
Flashes of lightning shook the peach trees, sending pale pink petals drifting down in a steady fall.
Kuwajima watched the two figures crossing blades with satisfaction.
Despite her personal feelings, Sui hadn't held back at all while teaching Kaigaku—if anything, she had been exceptionally thorough. With the Final Selection approaching, she had already guided the boy to the threshold of Breathing Techniques.
That alone was crucial.
Thunder Breathing users were rare, largely because of how difficult it was to master. Many swordsmen spent their entire lives without ever grasping it.
"Too weak."
Sui said calmly, flicking her blade in an arc and knocking Kaigaku's sword from his hands.
The gloom on the boy's face lasted only a moment before he forced a bright expression. He bowed, retrieved his blade, and took his stance again.
He couldn't calm himself during her training.
That night had left too deep a shadow.
She was like a demon—an ever-present fear that suffocated him.
Yet over time, he realized something—
He couldn't understand her at all.
Her disgust toward him was real.
Her knowledge of his secret was real.
And yet, she seemed utterly uninterested in him, focusing only on teaching him how to wield strength through repeated clashes.
What does she want?
The question haunted him every night, driving him to the brink of madness. The scar on his chest throbbed faintly, as if echoing that night.
"Kaigaku! Focus!"
Kuwajima's shout snapped him back.
The first thing he saw—
It was the slight curve of Sui's lips.
That faint smile, paired with her flawless face, was strangely alluring. Her closed eyes made her seem almost fragile.
But Kaigaku's nerves tightened instantly, veins bulging in his grip.
"Still can't grasp the First Form of Thunder Breathing?"
She smiled lightly.
He said nothing.
"The Final Selection is about to begin. This will be my last lesson."
"After this, you'll be the old man's responsibility."
Sui shrugged, still smiling.
To Kaigaku, that smile was unbearably fake.
Yet hearing she would leave—
He relaxed for just a split second.
Only a split second.
She vanished.
Gone from his sight.
Even Kuwajima's expression changed in shock—she had completely disappeared from everyone's perception.
"Thunder Breathing, First Form: Thunderclap and Flash."
A soft voice echoed.
Gentle, almost like poetry.
It sounded distant—yet right beside his ear.
—ting—
Kaigaku's ears rang.
His vision turned black and white.
A straight line of lightning cut through the world, silently slicing every petal along its path in two.
He had never seen lightning like this—
So quiet.
So beautiful.
All color drained from the world, leaving only black and white.
Kuwajima's heart lurched.
Was she going to kill him?!
This was the ultimate strike of Thunder Breathing—
An unrivaled, god-speed slash that could cut through anything.
Once drawn, it could not be stopped.
Not even by the wielder.
That blade would take Kaigaku's head.
Kuwajima lowered his stance, unleashing the same First Form—
But he was too late.
The strike had already fallen.
BOOM—!
Lightning descended and vanished into the peach grove.
Petals were thrown into chaos, obscuring his vision.
Faint arcs of electricity still danced among them.
By the time he could see clearly—
The wind from the slash arrived, scattering everything.
Sui stood there, her crimson haori fluttering, the embroidered cranes seeming to come alive—
But what echoed was not a crane's cry.
It was thundering.
Kuwajima froze.
The lightning faded from his blade.
Sui was smiling.
Her sword rested lightly against Kaigaku's neck.
Not a single drop of blood.
She could clearly see his expression—
Twisted. Pitiful.
Two conflicting emotions contorted his face, while his eyes were completely blank.
Did he break?
Sui withdrew her blade, lightly tapping his shoulder with the flat before sheathing it.
She knew Kuwajima was watching.
She returned his gaze with a faint smile.
Kuwajima realized something then—
Even his highest evaluation of her had fallen short.
He had taught many students. Some had even become Hashira.
But only Sui—
He looked at her quiet figure, eyes closed, serene—
She was the strongest.
Even relying purely on swordsmanship, she had already reached his peak from his prime.
What would she become in the future?
Perhaps…
The end of the Demon Slayer Corps' long war was approaching.
Perhaps she would be the one to slay the Progenitor Demon and end it all.
"The Final Selection."
Kuwajima said.
"I know."
She nodded.
"If necessary… you may open your eyes."
Despite fully trusting her strength, he couldn't help but say it.
"You worry too much."
Sui waved her hand, walking away lazily with her sword.
Then she paused, turning back slightly.
"No need to worry about him."
"That strike… was the peak of my training so far."
"Let him take his time understanding it."
"…Understood."
Kuwajima nodded.
Peak?
That was more than that.
That strike was nearly the ultimate expression of Thunder Breathing itself.
Her casual swordplay had already reached the level of his former peak.
It was both gratifying—and bittersweet.
Gratifying, that Thunder Breathing would live on.
Bittersweet, that his era had truly passed.
The future belonged to the young.
He glanced at Kaigaku, who still stood frozen, likely lost in that dazzling strike.
"Old man, make something to eat—I'm starving!"
Sui called out.
"…Coming."
He followed after her.
Only when they disappeared completely—
Did Kaigaku move?
The moment he did, his body collapsed, falling limply to the ground like mud.
His eyes were empty.
He picked up a fallen petal.
Split cleanly in two—
The cut surface is smooth as a mirror.
Kuwajima hadn't faced that blade directly.
He didn't know—
That strike had truly carried killing intent.
In that moment—
She had intended to kill him.
Kaigaku didn't know why she changed her mind.
Perhaps it was just a warning.
A reminder—
That even with a former Hashira beside him,
Taking his head would be effortless.
But none of that mattered now.
The Final Selection was beginning.
That damned woman—
Was finally leaving.
He cursed her in his heart—
Yet even here, alone in the wilderness,
He didn't dare let a single word escape his lips.