The smell of smoke lingered in the air.
Not thick, not suffocating, but persistent, curling into his nostrils with the faint bitterness of burnt wood and something… metallic.
Ren Aoki slowly opened his eyes.
Dust floated in the sunlight filtering through jagged holes in the ceiling. It hung in the air like snow trapped in amber. For a fleeting moment, he wondered if he was dreaming.
Then his fingers brushed the cold, gritty floor beneath him. Reality hit.
"…Where am I?"
The voice was barely above a whisper, hoarse and unfamiliar to his own ears.
He tried to sit up. Pain radiated through his limbs—muscles stiff from unnatural angles, joints screaming—but he ignored it. A slow blink cleared his vision. The ruins stretched out before him: shattered pillars, collapsed walls, charred beams. Ash floated lazily in the air, settling like snowflakes over the wreckage.
A mansion. Not just any mansion—his mansion. The one from memories he barely remembered owning. And yet… it didn't feel like his. Not anymore.
His mind blanked for a moment.
Then flashes returned:
A narrow room, glowing screens, a life lived quietly, almost invisibly.
A spacious hall, servants bowing, the sharp gaze of a man he called father, the gentle smile of a woman he called mother.
And the name, humming inside him like a half-forgotten song: Ren Aoki.
He clenched his fingers. "…Who am I?"
Two lives tangled in his mind. One was a lonely boy from another world, the other—a young master from a wealthy family whose life had ended in fire.
Ren closed his eyes briefly. When he opened them, his gaze was sharper. Deliberate. Calm.
"I see."
The words slipped out without effort. "…I'm Ren Aoki."
The wind stirred. Ash drifted across the floor.
Then memory struck like a blade:
Screams, red and bright in the dark.
The heat of fire licking at his skin.
Blood, spilling and staining everything.
A figure, pale and human-shaped, grinning with all the wrongness of a demon.
Ren's chest tightened. "…So that wasn't a dream."
He lowered his head. Not a single tear came. Just silence.
Another memory followed: a man in a dark uniform, sword in hand, calm voice cutting through chaos:
"You're the only survivor."
Ren exhaled slowly. "…Saved by a Demon Slayer, huh? Figures."
He looked up at the fractured ceiling. Blue sky shone through the cracks, stark against the blackened ruin.
A thought from his old life sparked in his mind, absurd yet fitting:
Demon Slayer. Really? Couldn't I get something normal? Like… accountant? Yeah, great. Nine-year-old me crunching numbers instead of dodging bloodthirsty monsters. Perfect.
His lips twitched. No sound escaped.
"If this really is that world…" He paused, gaze sharpening. "…Then swords are the only way to fight demons?"
The question lingered.
No one answered.
Ren tilted his head. "…Inefficient. Very inefficient."
A faint, mechanical voice whispered inside his mind.
[Techno Genesis System — basic module connected.]
Ren blinked. Curiosity, not fear. "…System?"
No heavenly glow. No booming proclamation. Just the soft certainty of a program quietly running in the background.
[Basic support functions available.]
[Advanced modules locked.]
Ren stared into the distance. The ruins offered nothing but the scent of smoke, the crunch of ash underfoot, and the faint rustle of wind through the broken roof.
He sighed. "…So I really did get isekai'd. Figures."
Another thought, absurd and childish, slid into his mind:Couldn't I at least wake up somewhere warm? Tokyo apartment? Pizza on the counter? No, gotta get charred mansions and murder demons instead. Lovely.
The wind blew again, carrying the faint tang of iron. Ren stood slowly. His small frame seemed fragile among the ruins, but his eyes were steady, alert, calculating.
He looked down at the ashes beneath his feet. "…Fine."
Something deep inside him had already shifted. Something old, hidden, waiting.
"If demons exist in this world…" His fist clenched. "…Then I'll deal with them my way."
Somewhere far away, in the shadow of the forest outside Tokyo, a creature stirred. Its instincts screamed. Something alien, unnatural, had appeared. Dangerous. Not human. And it did not belong here.
Ren stepped carefully over the blackened remains of the mansion, the soles of his shoes crunching against brittle ash. His eyes scanned every shadow, every angle, as if the walls themselves might harbor something alive.
The wind whispered through broken beams, carrying with it a metallic tang that made his nose twitch.
Well, this is cheerful. Ren muttered in his head, a faint smirk tugging at the corner of his lips. First day in a new world, first encounter with demons, and my house is basically an open-air oven. Perfect.
He knelt, running his fingers over the scorched floor. Ash clung to his skin, cool and gritty. Memories pressed against him again—not the mundane past of his previous life, but the one that had ended here, in fire and blood. He remembered the man in uniform, the calm voice, the words that had saved him.
"You're the only survivor."
Ren exhaled slowly. The statement was simple, yet it carried weight he had yet to fully understand. Surviving was one thing. Thriving, another.
A faint scraping sound made him freeze, barely audible over the wind. He tilted his head, instincts sharp despite his small frame.
Not human.
His pulse quickened—not with fear, but with the thrill of calculation.
Ren crouched, observing. A shadow moved near the edge of the ruins. Small, agile, almost too quiet. His eyes narrowed.
So the game begins.
He slid to the edge of the clearing, keeping his movements deliberate. Ash swirled beneath his steps, but he adjusted carefully, minimizing sound. The shadow flickered again—a small creature, humanoid but twisted, its limbs disproportionate. Its skin reflected sunlight oddly, though the sun was high; its eyes glinted with a strange hunger.
Ren's mind ticked. Definitely not friendly.
[Techno Genesis System — observation module active.]
He ignored the voice in his head at first, preferring to rely on his own senses. But curiosity nudged him.
[Creature detected: hostile entity. Estimated danger level — moderate.]
Ren raised an eyebrow. "Moderate?" He whispered. "I hate vague descriptors."
The creature tilted its head, sensing him now. Its gaze, sharp and predatory, locked on his figure. Muscles tensed. Ren's fingers itched toward the pocket of his coat, imagining the weight of a sword he didn't yet possess.
Patience, he reminded himself. No stabbing blind yet.
A faint chuckle escaped him, absurd and out of place: If I get eaten today, I'll haunt someone as a ghost. Preferably the one who designed this 'moderate danger' warning.
The creature leapt.
Fast. Its movement blurred, limbs whipping like jagged knives. Ren dodged instinctively, rolling to the side, eyes locked on the pattern of its strike.
Okay, fine. Not entirely moderate.
He didn't panic. He observed. Every lunge, every angle, every twitch of muscle was data. His previous life had trained him in observation and logic, though not in combat. Still, that experience was enough to give him an edge—thinking faster than instinct.
[System suggestion: deploy defensive module.]
Ren ignored it. He didn't need a crutch yet.
Instead, he ran a mental calculation. Distance, angle, timing. One step to the left, a slight lean, and the creature's claw tore through empty air. Its momentum carried it forward, forcing it to spin awkwardly mid-air.
Ren smirked. See? I'm already outsmarting it.
But then he stumbled slightly, caught off guard by the creature's next move—quick, unpredictable, erratic. He rolled again, scraping his elbow against ash. Pain flared but he didn't notice it. His mind was too busy analyzing patterns.
Okay… maybe it's smarter than it looks.
He stood, brushing ash from his clothes, keeping his posture casual. The creature hesitated, its predatory gaze flicking around, calculating him in turn.
Ren tilted his head, playful thoughts intruding:I wonder if it has a name. Something classy, like… Sir Fang. Or maybe Mr. Sneaky McClawface.
The creature hissed, unamused by his inner monologue.
Ren laughed quietly to himself. A low, soft sound that belonged to someone far older than nine, despite the childish humor in his mind.
He began to circle, matching its movements. Every twitch, every glance, every subtle shift in its balance was catalogued.
Patterns emerged. Weaknesses became visible.Predictable. Surprisingly predictable.
The creature lunged again, faster this time.
Ren sidestepped and tripped it—not with brute force, but with a precise shove that used its momentum against it. It crashed into a broken wall, leaving a dent but not enough to incapacitate it.
Ren sighed. Not yet perfect. He could improve.
[System update: combat experience +1.]
He blinked at the subtle notification. "Experience points, huh?" he muttered. "Guess it's like leveling up… but painfully slower than a video game tutorial."
The creature recovered, circling warily now. It had learned something too—though Ren doubted its intelligence matched his. That was fine. He liked challenges that forced thinking, not brute strength.
Okay, lesson one, he thought. Observation beats panic. Mocking the enemy beats boredom. And apparently… knocking things over counts as tactical genius.
Another wind gusted through the ruins. Ash swirled, eyes gleaming, teeth bared. The creature lunged again, faster, angrier. Ren sidestepped, letting its momentum carry it past. He grabbed a fallen beam—not to swing, but to trip it subtly. It crashed into the rubble again, yelping.
Ren exhaled softly, crouching, watching. He could have ended it in a strike, easily. But observation mattered more. Understanding mattered more. Killing came later, if necessary.
[System suggestion: initiate offensive module.]
He ignored it again. The system was useful, but he was learning something more important: patience, calculation, improvisation.
The creature hissed, frustrated, circling him slowly. Ren mirrored its movements, playful commentary rolling in his head:
Come on, Sir Fang… I mean, Mr. Sneaky McClawface. Show me your best trick. I've got all day.
And for the first time since waking in the ashes, Ren smiled—not out of joy, but anticipation.
The battle wasn't over. It had barely begun.
But he had already learned something vital: in this world, intelligence and observation were weapons too.
And Ren Aoki had plenty of both.