After finishing her duties at the Butterfly Mansion, Kanao left the mansion with Ushiwakamaru.
Okita had received a mission and was absent.
The two headed to Okita's vacant mansion.
As soon as they entered the mansion's bedroom, Ushiwakamaru's demeanor changed abruptly.
Her playful eyes calmed down, then soon dyed a deep red like an abyss.
What flowed from her mouth was not Ushiwaka's voice, but the solemn tone of Akira, the Night Emperor.
"You've worked hard, Kanao."
Akira, possessing Ushiwakamaru, led Kanao to the bed.
Creak.
Along with the grotesque sound, a wriggling change occurred in Ushiwakamaru's lower abdomen.
Akira's cells manipulated Ushiwakamaru's body, manifesting a male organ—massive and ferocious like Akira's own—on the female form.
"Father..."
Kanao's eyes grew damp with heat.
Without hesitation, she threw off her clothes and clung to Akira (Ushiwakamaru).
In the room lit only by moonlight, their bodies entwined. Moans and the sound of flesh clashing filled the silent mansion.
Amid the storm-like act of intimacy, a conversation unfolded.
Kanao, moaning in rhythm with Akira's thrusts, asked about Tanjiro and his companions.
"Hah...! Father♡ What will you do about the demon named Nezuko? You seemed quite... interested in her..."
"Hmm..."
Akira answered while piercing Kanao's depths.
"Nezuko is indeed a beauty. Her potential as a demon is excellent too. Above all... it's a waste to leave her be. Besides, in the original fate, she'd end up with that pathetic Zenitsu."
"Ze, Zenitsu... that noisy yellow-haired boy? Ugh...♡! That's absurd... Nezuko deserves better..."
Kanao frowned in disgust.
"Shall I tell you another interesting story?"
Akira lifted Kanao's chin and grinned mischievously.
"You're the same, Kanao. If I hadn't intervened, you were fated to end up with Tanjiro... that arrogant boy."
"...What?"
Kanao's eyes wavered. Herself with that boy who only smelled things well and was stubborn?
Married?
She immediately shook her head and buried her face in Akira's chest.
As if she didn't even want to imagine such a terrible fate.
She lowered her body and took Akira's manifestation on Ushiwakamaru's form into her mouth.
She sucked in a breath. "I don't want it... I only have Father... I don't need such a fate... Hnn..."
Kanao used her tongue to stimulate Akira.
It was an act akin to a vow, declaring herself solely Akira's possession, needing no man other than him, her father and lover.
"I know. The fate has already been twisted."
Akira smiled contentedly, seeing through Kanao's mind.
"Nezuko is on hold for now."
Kanao wiped the saliva from her mouth and looked up.
"Slurp, what about Aoi then♡ Aoi is... my most precious friend and disciple. I've educated her perfectly, so she'll serve you well, Master. I'm willing to offer her to you, Father."
"Hmm... Aoi."
Akira paused before speaking.
"Aoi, too, in the original fate, was destined to end up with that boar guy, Inosuke."
"...Huh?"
Kanao's expression froze coldly. First Zenitsu, now Inosuke? That ignorant, mannerless beast in a boar mask, with her precious, diligent Aoi?
"Father... you're joking too much. I can't give Aoi to such a beast. Absolutely not."
A cool killing intent flashed in Kanao's eyes.
The very idea of her precious people being entangled with such ones in the original fate was unpleasant.
"Then keep a close watch. Fate will struggle to find its place."
At Akira's words, Kanao resolved.
Tomorrow in training, she would thoroughly crush those guys, ensuring they dared not lay eyes on her friend and herself.
The next morning, at the Butterfly Mansion training ground. The air was much heavier than the previous day.
Kanao wore her usual smile, but behind it lay a killing intent sharp as a blue blade.
"Let's begin training."
It was not training but devastation. Kanao thoroughly toyed with Tanjiro and his companions in tag.
Not merely evading, but tripping them, shoving their shoulders, and slamming them humiliatingly to the ground.
"Gah! What, what's this! You're even faster than yesterday!"
Zenitsu screamed as he fell. Inosuke too was kicked into a corner by Kanao, groaning.
"Inosuke, don't even come near Aoi."
Kanao stepped on Inosuke and whispered coldly. Inosuke, clueless, was just terrified.
Tanjiro desperately chased Kanao, but the result was the same. What bewildered him most was the smell.
Sniff.
'This smell...?'
A peculiar fragrance mingled with the sweat from Kanao's vigorously moving body.
Sweet yet bloody, sticky yet primal... similar to the smell he'd caught when opening his parents' bedroom door the morning after his father spent the night with his mother.
But much denser, a oppressive male scent permeated her entire being.
'Yesterday... what happened? She seems angry, but the smell is oddly... satisfied.'
Tanjiro, confused, was eventually knocked out by Kanao's hand chop.
"So weak, pitifully so."
Thus, ten hellish days passed. Zenitsu and Inosuke, hitting an overwhelming wall, often gave up training and ran away.
Even when Aoi scolded them, they didn't listen. But Tanjiro didn't give up. From morning till night, he devoted himself to training to defeat Kanao.
"Tanjiro-san, wait a moment."
One night, the three girls of the Butterfly Mansion—Sumi, Kiyo, and Naho—approached Tanjiro, who was training alone.
"Tanjiro-san, you're really working hard. So we'll give you a hint."
The girls surrounded Tanjiro and cautiously spoke.
"Are you maintaining Total Concentration Breathing... all day long?"
"Huh? All day?"
Tanjiro's eyes widened.
"Yes. It's called Total Concentration: Constant. Maintaining your breathing without rest, even while sleeping or eating, to push your base stamina to the limit. Kanao-sama, Aoi-san, and the Hashira all do it."
"And... you need to be able to blow this up."
The girls presented a massive gourd the size of a child's torso. It was hard as a rock.
"Th, this? With my mouth?"
"Yes. Increasing your lung capacity with breathing power to burst the gourd. Kanao-sama can burst a gourd as big as herself in an instant. Even Aoi-san, though she doesn't fight, can do this much after learning from Kanao-sama."
Tanjiro was shocked. The world of breathing he knew was just the tip of the iceberg.
Even Aoi could do what he couldn't. From that day, Tanjiro began an ascetic practice to master the Constant.
Enduring pain that felt like his lungs would tear, he struggled to maintain his breathing 24/7.
Even while sleeping, the girls beat him with clubs to keep his breathing steady.
On the 15th night of rehabilitation training. Tanjiro climbed onto the Butterfly Mansion roof, sitting in lotus position and meditating.
His lungs felt ready to burst, veins bulging, but he forced himself to regulate his breath and focus.
Tap.
Someone ascended the roof without a sound. A silhouette standing against the moonlight. The girl with butterfly ornaments, Kanao.
"...Still at it?"
Kanao asked with an expressionless face.
Tanjiro, struggling not to break his breathing, opened his eyes.
"Yes... I want to... defeat you, Kanao-san. And... because I have something to protect."
"Defeat me..."
Kanao sat beside Tanjiro. Her hair fluttered in the night breeze.
From her, that peculiar, faint scent of Akira wafted again.
"You're a strange one. It'd be easier to give up."
"If I give up... I can't protect Nezuko."
Tanjiro's earnest eyes. The destined partner Akira had mentioned. He was indeed kind and sincere.
But Kanao's heart didn't beat faster. Already, her everything was consumed by the immense being that was Akira.
"Keep trying. If you break the gourd... I'll face you again."
"Yes. Ah, um. There's something I want to ask."
"What is it?"
"Have you ever heard of Hinokami Kagura?"
Tanjiro asked about the breathing he uses, the Kagura passed down in his family.
"No, not at all."
"Eh. Then, what about Fire Breathing!"
"...There's Flame Breathing, but no Fire Breathing. However, there's a rule that Flame Breathing must never be called Fire Breathing. Do you know why?"
"Why?"
Kanao grinned, bringing her face close to Tanjiro's.
"Because of a breathing with the same pronunciation. Sun Breathing."
"Sun Breathing?"
"Throughout history, many breathings have been passed down in the Demon Slayer Corps. Water, Flame, Wind, Thunder, Stone. And Mist, as old as those five. Then Flower too. And the ones developed by current Hashira: Sound, Serpent, and the Insect and Love that I use."
'Insect, Love? All new to me. Like Inosuke's Beast Breathing?'
Tanjiro was puzzled by the unfamiliar breathings.
Leaves blew in the wind as Kanao, back to the moonlight, spoke to Tanjiro.
"And the origin of all those breathings. That's Sun Breathing. All breathings are derivatives of Sun Breathing."
"Derivatives of Sun Breathing?"
"Yes. It's said to be the breathing used by the first swordsman to wield breathing. That swordsman's blade was a bright red sword. And he was the strongest in the Demon Slayer Corps, almost killing Muzan."
Tanjiro was startled by the mention of almost killing Muzan. Seeing this, Kanao smirked and shared more information known only to her in the Corps.
"But Sun Breathing has fallen into disuse. He apparently had many disciples, but they all died. Strangely, though, a Nichirin Blade color appears in every era that can't use that breathing at all. Namely, black."
"!!!"
"Tanjiro. Perhaps your Nichirin Blade is black because you use that Hinokami Kagura breathing? Hinokami, doesn't it mean god of the sun?"
"I, my family is a charcoal-making house, so we thought it meant god of fire. My father too."
"Maybe that Hinokami refers to that first swordsman. Tsugikuni Yoriichi. And that swordsman wore earrings like your hanafuda ones."
"Th, these earrings?"
"Ah, now that I think of it. Our Demon Slayer Corps has a descendant of that first swordsman."
"Really!?"
"Tokito Muichiro. The Mist Hashira. Of course, he can't use Sun Breathing. He's definitely a descendant of the Tsugikuni family, but whether he's Yoriichi's direct descendant is unknown. There's no record of him having children, after all."
As if her service was over, Kanao slowly rose.
"If your breathing is truly Sun Breathing, be careful. If it's known that someone using a breathing disused for centuries has appeared, Muzan won't stay idle. He wouldn't spare someone using the breathing of the swordsman who almost killed him. Perhaps the reason black blade wielders are short-lived is because they're targeted by Muzan."
She left a meaningful warning and, like a butterfly taking flight, kicked off the roof and vanished into the distance.
"Sun Breathing. The first swordsman? Something difficult. Is my family his descendant? But I've never heard that name."
Tanjiro tilted his head, repeating Kanao's words. He felt his head overheating from the sudden wave of information.
And, Kanao watching from afar.
"Who knows. Using Sun Breathing might lure out Upper Moon One. I hope so. Father seems to want Moon Breathing."
The ultra-rare breathing with only one user in history.
Akira was showing interest in that Moon Breathing.
The reason.
Shinobu's Nichirin Blade color was suited for Moon Breathing.