"... 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.
"This is it… This is my path. My own path. Something that exists before and after fire. Smoke… Smoke Breathing Technique (Kemuri no Kokyū)"
The realization struck him like lightning. It wasn't the path of flames, but a path of haze, concealment, and lingering presence. Harder. Unfamiliar. But because of that, Kagerou's lips curved into a smile.
"…Kemuri no Kokyū"
"Kage-kun"
That gentle voice, the one that always steadied him, pulled him from his thoughts. Kagerou turned, startled.
"…Mother, you're not asleep yet?"
Ruka hummed softly, not answering right away. Instead, she came closer and sat beside him on the engawa. Without a word, she patted her lap.
"…Eh?"
*pat pat*
Kagerou froze, but he knew exactly what she meant. Before he could resist, Ruka's hands tugged gently at his shoulders, guiding him down until his head rested against her lap.
"R-Ru- Mo-Mother?!"
"Kage-kun," she said again, her hand brushing through his damp hair.
"Y-Yes?"
Her gaze softened, her voice low and calm.
"Why is it… that you want to be strong?"
Kagerou froze, her question slipping into his heart like a blade, not sharp, but heavy. He stared up at her face, softened in the moonlight, her gentle hand brushing through his damp hair.
Why?
The words echoed inside him.
For as long as he could remember, strength wasn't something he simply wanted. It was something he needed.
For someone who once had nothing and then, abruptly, gained everything, the Rengoku household was more than family. Shinjuro had been the first hand to pull him out of the ash. Ruka and Kyojuro were anchors that proved he existed in this foreign world. They were proof that he was no longer alone.
And because of that, quiet hunger that gnawed at his bones each time he saw Shinjuro return wounded, each time he heard Kyojuro's laughter masking worry, each time Ruka coughed quietly in the night when she thought no one noticed.
His lips trembled before sound came.
"Because… I don't want to lose anything," he whispered. "Not Shinj- Father. Not Kyojuro. Not you, Mother. I don't want to stand by and watch, powerless, while someone I love struggles. If I must walk a strange path to protect what I have now, then I'll become the strongest. If killing demons is what it takes to keep this warmth, then... No demon will ever walk this world again. Is that a good enough reason, Mother?"
Ruka's hand stilled for a heartbeat, then resumed its slow rhythm over his hair. Her eyes softened, but shadows lingered in them.
"Kage-kun…" she murmured. "You're only five. Children your age should dream of games, not battles… not killing"
Kagerou clenched his small fists against his chest. His gray eyes glinted with a determination too heavy for his years.
"I can't, Mother. I don't know why, but… it feels like I was meant for this. Even if I close my eyes I can see it... the sword, the breath, the fight. If it means protecting everyone, then I'll carry it. No matter the weight"
Silence lingered, broken only by a night breeze and the distant hum of crickets. Finally, Ruka sighed, a sound braided with sorrow and pride. She bent and pressed a gentle kiss to his forehead.
"Then promise me one thing, Kage-kun," she said, gentle but firm. "Don't lose yourself. Even if you walk a path stained with blood… don't let it smother the boy who smiles, the boy who calls me 'Mother'"
Kagerou's chest tightened, but he nodded. "…I promise"
Her hand lingered on his cheek, warm against his cool skin. "Good boy," she whispered. "Now rest. Tomorrow, you'll need your strength… my little swordsman"
Kagerou closed his eyes, but even as sleep tugged at him, the faint curl of smoke slipped from his lips again, twisting in the moonlight like a quiet oath.
Not long after Kagerou drifted to sleep, Ruka stayed, her fingers tracing lazy circles through his hair. Footsteps approached quietly.
Shinjuro sat beside her, breathing in the soft house-scent, the faint soot, the tea, the moon-washed wood.
"You heard that, Shinjuro-san?" Ruka asked, eyes on the sleeping boy.
Shinjuro's expression softened as he looked at Kagerou, pride mingling with a blade-edge of concern.
"That is a strong resolution," he said. "But smoke breathing? There's no record of anyone using it. It will be an unknown road. Difficult. Dangerous"
Ruka's gaze did not waver. "And you will be there, beside him?"
Shinjuro's jaw set. He glanced at their son curled into sleep, then back to Ruka. "We will. Together. You, me, and Kyojuro, everyone. Because we are family. That's what family does"
Ruka smiled, small and sure, and laid her head against his shoulder. Outside, the night kept its quiet watch. Inside, by the slumbering child who breathed out smoke, a promise had been sealed, soft, steadfast, and ready to stand against the dark.
--------
- One year later.
Kagerou had turned six, and little Kyojuro was already three, a bundle of fire and laughter.
In that one year, Kagerou had carved his own path. Kemuri no Kokyū, Smoke Breathing had taken shape under his hands, two forms born through relentless training. Two forms might seem little, but for a Breathing Style born from nothing, it was a miracle.
That same year, after Ruka's words beneath the moon, Shinjuro began to change. He took fewer missions, passing along anything another Demon Slayer could handle. He only stepped in when the danger demanded a Hashira's presence. The rest of his time, he poured into Kagerou. Into shaping him.
Now, in the backyard of the Rengoku residence, father and son stood across from one another.
"Kage-nii!" Kyojuro's voice rang bright as sunlight. "I believe you can defeat Father!"
Beside him, Ruka pressed a hand to her lips to hide her laughter.
Kagerou's gray eyes flicked toward Shinjuro, calm but mischievous. "You hear that, Father? Seems like I've already won"
Shinjuro's lips twitched. "…Shut up," he muttered, though the faintest scowl betrayed more than irritation. He hated how easily Kyojuro's little heart leaned toward Kagerou, but he also couldn't deny the warmth that filled his chest at the sight.
He rolled his shoulders, expression hardening again. "Enough talk. Come at me. Show me this new form of your Kemuri no Kokyū."
The backyard went still. Kagerou tightened his grip on the wooden blade, smoke already curling faintly from his lips.
And the fight (Training) between Father and Son is about to begin.