A day had passed. In the depths of Infinity Castle, cracks began to form in the red cocoon placed in the isolated area.
Crack, crackle.
Finally, the cocoon shattered, and Kokushibo walked out. Six eyes, a red kimono, a sword at the waist.
His appearance wasn't much different from before. But the aura he exuded was on another level.
Muzan's murky aura was gone, leaving only a pure and overwhelming fighting spirit.
"...Amazing."
Kokushibo muttered as he clenched and unclenched his hand.
"My body feels as light as a feather. Yet the inherent power within is heavy enough to move mountains. I sense the sun's energy, but I don't burn. Is this... evolution?"
Following Akira's orders, Nakime had intentionally allowed sunlight to pour into Infinity Castle. Having received a large amount of Akira's blood, she had achieved sun resistance as a precaution.
It was a special measure, similar to what had been done for Mitsuri, Kanae, or Shinobu.
"You seem satisfied."
Akira approached with his arms crossed.
"You look the same, but every single one of your cells has been replaced with mine. If you want, you can change your appearance too. Like Muzan did."
"Change..."
Kokushibo closed his eyes and focused his senses. He felt his cells rearranging.
Slither.
Inside his body, muscle tissues shifted, and his six eyes reduced to two. Then Akira tossed him a hand mirror.
The reflection showed the very image of Tsugikuni Michikatsu, the samurai who lived during the Warring States period 400 years ago.
"...It's been a long time. This face."
Kokushibo stroked the mirror, lost in sentiment for a moment. He tried to revert to his original demon form. But then.
Crunch! Crack!
The sound of bones adjusting echoed as his physique changed drastically. His shoulders narrowed, and his hips widened.
His muscular chest softened and swelled. His rough jawline became slender.
As the smoke cleared, what appeared was a peerlessly beautiful swordswoman with six eyes.
The curves revealed through her kimono were alluring, and the aura she emitted was chillingly beautiful.
"...Huh?"
Akira's jaw dropped.
"Hey, why did you turn into a woman? I told you not to do that!"
The female Kokushibo looked down at her body and replied calmly.
"Do not be angry, Master. This is not mere caprice."
She drew her sword and lightly swung it through the air.
"A female body has advantages that males cannot possess. Namely, flexibility."
Kokushibo bent her waist like a bow as she thrust her sword. The range of motion in her joints had become strangely wide.
"In the history of the Demon Slayer Corps, Flower Breathing is famous as a sword style that utilizes the unique softness of women. By adding feminine flexibility to Water Breathing, it creates unpredictable trajectories. Male swordsmen rely on brute strength to overpower, but they cannot match the irregularity born from this flexibility."
"But did you really need to become a woman? With a demon's regenerative power, you could just twist your joints."
"That would be forced contortion. It disrupts the flow and feels unnatural."
Kokushibo explained earnestly.
"However, a female's skeletal and muscular structure is inherently flexible from birth. With this body, I can make the slashing trajectories of Moon Breathing even more varied and intricate."
The epitome of a sword style utilizing feminine flexibility was Love Breathing.
It required immense muscular strength like Mitsuri's, along with her speed and exceptional flexibility.
In terms of pure strength, people like Tengen or Gyomei might manage, but they lack Mitsuri's flexibility. In fact, their muscular bodies might make them less flexible compared to other Hashira.
Tengen, trained as a ninja, is relatively flexible, but even he cannot replicate the flexibility inherent in Love Breathing.
Kokushibo's words made sense. The advantages of flexibility are vast.
Meanwhile, Akira was most bothered by the fact that she had become a woman and was now speaking politely—in his standards, femininely polite.
"...Yoriichi didn't mix flexibility into his sword, but as a genius, he must have been far more flexible than Flower Breathing swordsmen. He could reproduce any Breathing style. Since I cannot achieve that, I seek to glimpse that realm through physical change."
Akira recalled Inosuke, who uses Beast Breathing.
That feat of dislocating joints to wield them like whips seemed impossible even for Yoriichi, but Kokushibo's logic held merit.
A body possessing both strength and flexibility could be another key to approaching the pinnacle of martial arts.
Kokushibo adjusted her attire and added.
"And, Master."
Her six eyes gleamed with a seductive light.
"If you feel any lust toward me like this, you may act as you wish. I am a servant who drank your blood. Offering my body to the benefactor who opened a new realm is a price I, as a warrior, must bear. Even if ravaged in this maiden's form... I have prepared myself."
"No, that's enough."
Akira let out a deep sigh with a genuinely weary and disillusioned expression.
"How many times do I have to say it? I don't see you that way. No matter how beautiful you've become, my mind is stuck with the image of a six-eyed old man, so it doesn't work. Do whatever you want. You're hundreds of years older than me anyway."
"..."
Kokushibo's brow furrowed slightly.
'It doesn't work...? Am I not attractive? In my human days, I was quite the charmer...'
Unconsciously, Kokushibo felt a strange blow to her pride.
"Enough of that. Let's test the performance of your new body."
Akira clapped his hands.
"Come out. Kanae, Shinobu, Mitsuri, Tsubaki."
At Akira's call, four women descended into the training ground.
"Upper Moon One... or should I call you Kokushibo-sama? You've truly become beautiful."
Kanae smiled and drew her sword.
"Spar with them. I want to see firsthand how much you've changed and what your current strength is like."
"Understood."
Kokushibo took her stance.
"Come at me."
Whoosh!
Mitsuri was the first to leap out. Her flexible sword whipped through the air, aiming for Kokushibo's neck.
"Love Breathing, Fifth Form: Swaying Love — Wild Claw!"
"Fast."
Kokushibo's body twisted oddly. She bent her waist backward to evade Mitsuri's sword while swinging her own upward from below.
Clang!
"Whoa?!"
Mitsuri retreated in surprise. Kokushibo's slashing trajectory had curved in like a serpent.
"Next!"
This time, the sisters Kanae and Shinobu attacked together.
"Flower Breathing, Second Form: Honorable Shadow Plum!"
"Insect Breathing, Dragonfly Dance: Compound Eye Hexagon!"
An elegant slash and a poison-imbued thrust surged simultaneously. But Kokushibo didn't panic.
She split her legs straight and pressed close to the ground to avoid the attacks, then spun in that same posture to release a slash.
"Moon Breathing, Sixth Form: Perpetual Night, Lonely Moon — Incessant."
Countless slashes stormed forth like a tempest.
But unlike the destructive slashes of before, they now carried a delicacy that curved smoothly, digging into the opponents' openings.
"Guh...!"
Kanae and Shinobu blocked with their swords and were pushed back.
Finally, Tsubaki stepped forward.
She employed the hybrid swordsmanship style that simultaneously uses Flower Breathing, Water Breathing, and plant-based Blood Demon Art.
"Here goes! Flower Breathing, Fifth Form, Blood Demon Art: Red Camellia — Decapitation!"
As Tsubaki swung her sword, nine consecutive slashes along with camellia petal-like strikes surrounded Kokushibo. If cut, camellias would grow from the wounds, draining her blood.
"Hoh... A fusion of Breathing and Blood Demon Art."
Kokushibo's eyes flashed. An ordinary swordsman would have had no escape, but she was different now.
Swish~
Kokushibo used her bone joints smoothly, twisting her upper body like a snake to slip through the fine gaps between the slashes.
"Moon Breathing, Third Form: Loathsome Moon — Annihilation."
Her sword didn't cut through the petal strikes but flowed along them, striking Tsubaki's sword. It was a movement like flowing water.
Tap.
Kokushibo's blade stopped just before Tsubaki's throat.
"...Match over."
Akira declared. The four swordsmen caught their breath as they stared at Kokushibo.
"Incredible... Not just strength, but your movements are so fluid. More than mine."
Mitsuri marveled.
"Indeed... The trajectories were hard to predict. This is Moon Breathing, and with added flexibility. It was already a threatening Breathing style to begin with."
Kanae nodded in acknowledgment.
Kokushibo sheathed her sword and looked at Akira. Her six eyes shone with confidence.
"You see, Master. This is the path of evolution I have chosen. The harmony of flexibility and strength. A martial pinnacle of both genders that not even Yoriichi reached."
Akira shook his head and chuckled wryly.
"Look at you, all smug and satisfied. Fine, I admit it. You've gotten stronger. But... don't come to my room at night. Got it? Stay in your male form around me normally. Somehow, it's a bit irritating to look at."
Instead of answering, Kokushibo simply smiled mysteriously.
In the training ground of Infinity Castle, a cool moonlight shone, heralding the birth of a new powerhouse.