Taisho Era – Year 3 (1914)
Location: The Outskirts of Azabu
A year had passed since the night of the pink sakura. At age six, Kai no longer looked like a mere child; his frame was lean and corded with dense, efficient muscle that defied the soft roundness of youth. His burgundy hair had grown longer, now tied firmly with a simple hemp cord, and the mark of his focus—a terrifyingly calm intensity—never left his eyes.
He stood in a clearing deep within the woods bordering Azabu, the morning mist clinging to his skin like a damp veil. In his hand was no longer a simple wooden stick, but a heavy training bokken weighted with iron cores, a custom piece he had commissioned from a blacksmith under the guise of "medical equipment for resistance therapy."
'Great Sage, initiate bio-rhythm synchronization. Target: 100% lung capacity utilization.'
[Notice: Total Concentration Breathing: Constant is currently active. Oxygen saturation in blood is optimal. Commencing "Dance of the Fire God" simulation. Warning: Heart rate will spike to 200 BPM. Please monitor internal temperature.]
Kai took a stance. His feet dug into the soft earth, grounding him. He closed his eyes, visualizing the charcoal-burner's dance he remembered from the future—or rather, the ancient past.
'Breath of the Sun, First Form: Waltz.'
He moved. It wasn't a swing; it was a revolution. The wooden blade carved a golden arc through the mist. The air around him didn't just move; it shivered. The sheer friction generated by his "Yoriichi-like Physique" combined with the breathing style caused the surrounding moisture to hiss into steam.
'Second Form: Blue Heaven.'
He spun, a 360-degree vertical slash. His muscles screamed, a searing heat blooming in his chest. This was the "Fire" he needed to master.
[Warning: Core temperature rising to 39.5°C. Muscle fiber micro-tearing detected in the deltoids. Recommendation: Adjust grip by three millimeters to distribute the centrifugal force.]
Kai gritted his teeth, sweat pouring down his face. 'Silence, Sage. If I can't handle the heat of a wooden sword, how will I handle the blade of a Hashira? Adjusting grip now.'
He corrected his form mid-motion, the pain dulling as the Great Sage optimized his nerve endings. He was a machine of flesh and will, rewriting the lost history of the sun.
"You're doing it again, Kai-kun. You're making the air taste like lightning."
Kai exhaled, a long, visible plume of steam venting from his lips. He didn't turn around; he already knew the rhythm of those footsteps. Mitsuri was standing at the edge of the clearing, her hair now reaching her mid-back, her own weighted wristbands—the ones Kai gave her—clinking softly.
"Lightning is a bit dramatic, don't you think?" Kai said, turning with a playful smirk. He was drenched in sweat, his chest heaving, but his voice was steady. "I was thinking more like a warm summer breeze."
Mitsuri walked over, dabbing his forehead with a silk handkerchief. She was six now, and her strength had become a local legend in her household. She had accidentally lifted a stone lantern last week while looking for a lost hairpin.
"A breeze doesn't snap thick branches just by passing them," she countered, pointing to a nearby maple tree where the air pressure from Kai's swing had stripped the bark. She looked at him with a mix of awe and a strange, budding concern. "Why are you pushing yourself so hard? You already have the money, the respect... everyone in Azabu says you're the 'Little Sage'."
Kai took the handkerchief, his fingers brushing hers. "Money and respect don't stop the shadows, Mitsuri-chan. I told you before—there are things in this world that eat peace for breakfast."
Mitsuri bit her lip. "The monsters you talked about? At dinner, my father said you were just being 'poetic.' But when I see you train... it looks like you're preparing for a war."
Kai looked at her, his expression softening. He couldn't tell her about Muzan Kibutsuji. Not yet. She deserved a few more years of innocence.
"I'm preparing so that you never have to," he said, tucking the bokken under his arm. "Speaking of which, how is your training going? Can you still move as fast with the weights on?"
Mitsuri's face lit up, her natural competitiveness surfacing. "Watch this!"
She suddenly blurred. In the blink of an eye, she was ten feet away, then back again, a gust of wind following her. Her speed was raw, unrefined, but terrifyingly potent.
"Impressive," Kai chuckled, though his inner mind was racing. 'Great Sage, analyze her movement.'
[Analysis: Mitsuri Kanroji's muscle density is approximately 8 times that of a normal human. Her flexibility is off the charts. Current potential: Upper Rank tier if properly trained. Suggestion: Introduce 'Love Breathing' foundations early? No, data suggests she must find her own rhythm. Suggestion: Teach her basic flexibility-based defensive forms.]
"But," Kai added, stepping into her space with a mischievous glint in his eye, "you're still leaving your left side open. If I were a 'monster,' I'd have stolen your mochi by now."
Mitsuri pouted, her cheeks puffing out. "You're the only one who can catch me! It's not fair. You're like a ghost."
"A handsome ghost, I hope," Kai flirted shamelessly, making her giggle and blush. "Come on. I've earned enough today to buy out half the confectionery shop. And I heard they have a new batch of honey-soaked chestnuts."
As they walked back toward the bustling streets of Azabu, Kai's mind remained split. One half was enjoying the simple joy of Mitsuri's chatter—she was talking about a kitten she'd found—while the other half was cold, calculating, and distant.
'Great Sage, status of the Demon Slayer Corps. Have we found any traces of their presence in Tokyo?'
[Notice: No official sightings. However, rumors from the northern charcoal-burning regions suggest 'unexplained disappearances.' The Kamado family remains undisturbed for now. Timeline check: We are approximately 8 years away from the Main Plot.]
'Eight years,' Kai thought. 'Eight years to reach a level where I can kill Upper Ranks in my sleep. Eight years to ensure that when the Flame Hashira meets his fate, I am there to extinguish the fire that kills him.'
"Kai-kun? You went quiet again," Mitsuri said, swinging their interlaced hands. "You do that a lot lately. Like you're looking at something very far away."
Kai stopped and looked at her. The sun was setting, casting a golden glow over her features. She was his "wifu," his favorite character, and now, his closest friend. The thought of her dying in the final battle against Muzan, her body broken and her heart heavy, sent a surge of cold fury through his veins.
"I was just thinking," Kai said, his voice low and fervent, "about how lucky I am to be the one standing next to you. And how I'll never let that change."
Mitsuri stared at him, her green eyes shimmering. At six years old, she didn't fully grasp the depth of his words, but she felt the protection radiating from him. She leaned in and planted a quick, messy kiss on his cheek before running ahead, laughing.
"You're a flirt, Kai-kun! Catch me if you want those chestnuts!"
Kai stood there for a moment, touching his cheek. A small, genuine smile broke through his warrior's mask.
'Sage,' he thought, his spirit igniting. 'Double the training repetitions tonight. I want to see the 'Clear Blue Sky' form before the month is out.'
[Accepted. Commencing optimization of sleep-cycle training. You will practice in your dreams.]
Kai took off running, his footsteps silent, chasing the pink-haired girl into the golden twilight of a world he was destined to save.