In the front yard of the Rengoku residence, the sound of wood cutting through the air rang again and again.
*Thwack. Thwack. Thwack*
Kagerou's wooden blade rose and fell in a steady rhythm, each swing counted under his breath. "…132. 133. …150"
Sweat streamed down his face, soaking into his ponytail, dripping onto his bare shoulders. For someone so young, his breath was unnervingly calm, his form unwavering. Anyone watching would never believe he was only five years old.
"…Kage"
The familiar voice broke through his focus. He halted mid-swing, lowering the sword. Turning, he found Shinjuro standing in the doorway, clad in his simple white kimono.
"Shinjuro-san. Good morning," Kagerou greeted politely, wiping his brow. "How was the mission?"
"Nothing worth talking about," Shinjuro replied with a dismissive wave. "Just some low-level demon puffed up with false bravado. Went down fast." He paused, his sharp eyes narrowing as he studied the boy. "But Kage…"
"…?"
Shinjuro's brow arched. "It's been a year now, hasn't it? You call Ruka 'Mother.' So why am I still just 'Shinjuro-san'?"
Kagerou stiffened. Slowly, he set the wooden sword aside, scratching the back of his head. "…Ah… I just feel more comfortable that way. Unless… you'd prefer I call you Father?"
Shinjuro chuckled, shaking his head. "No need to force it. If 'Shinjuro-san' is what you're comfortable with, that's fine. I won't push you"
His smile deepened as his eyes roamed Kagerou's frame, from his long limbs to the sharpened lines of his young face. "Still… are you really only five years old?"
"…What do you mean?" Kagerou asked, blinking.
"Nothing," Shinjuro laughed roughly, though his eyes lingered with something unspoken. "It's just hard to believe a child could grow this much in one year"
Hearing that, Kagerou didn't show much reaction. It wasn't the first time Shinjuro had teased him like that.
"…Where's Mother?" he asked instead.
"Ruka? She went to the market with Kyojuro. Why don't you go with them?"
At those words, a chill ran down Kagerou's spine. He remembered the last time he tagged along, it hadn't been the market that overwhelmed him, but the swarm of young girls. With his striking features and graceful bearing, he wasn't just handsome for his age, he was captivating. Too captivating. He could still recall being surrounded, tugged at, and questioned endlessly. Just the memory made him shudder.
"…No. Never again," Kagerou said flatly.
Shinjuro blinked, tilting his head in mild confusion.
"…Alright then," he finally said.
"Before that, Shinjuro-san… when will I learn the Breathing Technique?" Kagerou asked. He knew there was a right time for everything, but after a year of nothing but basics, his patience was starting to fray. He felt ready.
Shinjuro fell silent, his sharp eyes narrowing as he studied Kagerou's posture, his chest, his stance, his entire being.
"Hm…" He folded his arms, lost in thought. In truth, the ideal age to begin Breathing was around ten, when the body had reached a balance of growth and adaptability. At that stage, the lungs could endure harsh strain, and the muscles could develop alongside training.
Younger than that, most children were simply too fragile. Their bodies would break long before their spirits did.
"…Five years old is still too early," Shinjuro finally said. But his lips curved into the faintest grin. "Still… looking at you now, I'd be lying if I said you weren't an exception"
Kagerou's gray eyes widened. "R-Really, Shinjuro-san?!"
Shinjuro chuckled, scratching his chin. "Yeah. But before you start, there's something you need to understand first, the origin of Breathing Techniques"
Shinjuro sat down on the engawa, gesturing for Kagerou to sit across from him. His tone shifted, still rough, but carrying the weight of a teacher.
"Listen carefully, Kage. Every Breathing Technique we use today… Every Flame Breathing form, every Water, Wind, Stone, Insect, Serpent, hell, even the stranger ones, they all came from one man"
He paused, as if measuring his words.
"His name was Yoriichi Tsugikuni. The strongest swordsman to ever walk this earth. He created the first Breathing Style, Sun Breathing. That was the origin"
Kagerou blinked, the name unfamiliar yet strangely heavy in his chest. "…Sun Breathing?"
Shinjuro nodded. "Yes. A technique that mirrored the sun itself, flawless and absolute. Too perfect for most people to master. So Yoriichi taught others to adapt it to their own strengths, their own bodies. From that, new Breathing Styles were born"
He lifted a hand and pointed to his own chest. "Flame Breathing, the style I carry. Water Breathing, Wind, Stone, Thunder. The five main styles that shaped the Corps. Every other Breathing Technique that exists… is just a branch of those"
Leaning forward, he jabbed a finger at Kagerou. "Remember this: Breathing isn't just swinging a sword. It's forcing your lungs, your blood, and your body to move in ways no ordinary man can. To breathe life into your blade until it burns, cuts, or freezes like the forces of nature itself"
Kagerou swallowed, wide-eyed. His heart thumped fast at the thought of such power.
Shinjuro smirked faintly. "You want to learn your own Breathing, Kage? Then you'll have to start with the foundation: total control over your breath. The first step of every Demon Slayer"
Kagerou's eyes lit up with determination. "Then… please teach me, Shinjuro-san! I want to learn Flame Breathing!"
For a moment, Shinjuro said nothing. He simply studied the boy, those gray eyes, sharp features, and that strange, unshakable aura that didn't quite fit a child of five. Finally, he gave a short nod.
"…Alright. If you're set on it, I'll start you on the basics of Flame Breathing. But listen, Kage, Flame Breathing isn't something you master overnight. It's about intensity. Power that burns bright and leaves nothing behind. You'll have to carry fire in your lungs"
Kagerou clenched his fists. "I can do it"
Shinjuro allowed himself a small smile at the boy's resolve. "…We'll see"
That day, Kagerou began his training in Flame Breathing. His chest ached as he forced his lungs to draw more air than he thought possible. His vision blurred, his ribs screamed, and sweat poured down his skin. But he endured. Again and again, he inhaled, exhaled, and repeated the harsh rhythm Shinjuro drilled into him.
Weeks passed. His body adapted, his breath steadied, his swings grew sharper. And yet, something felt wrong.
Each time he tried to ignite the fire Shinjuro spoke of, the flame flickered weakly, like smoke struggling to become fire. His body obeyed the forms, but his spirit resisted.
Late one night, as he sat on the engawa, exhausted and drenched in sweat, Kagerou exhaled a long breath. Instead of flame, a pale, curling haze slipped from his lips, thin smoke twisting in the moonlight before fading into nothing.
He stared at it, stunned.
"…Smoke?"
For the first time, Kagerou felt the faint tug of something deeper, something that wasn't fire, but had been with him all along.