Within a week, the rumors had reached the highest levels of the Demon Slayer Corps. A nine-year-old boy, a survivor of the Aoki massacre, was performing "alchemical miracles" that bypassed traditional breathing techniques.
Three Hashira—Shinjuro Rengoku (the Flame Hashira), a young Tengen Uzui, and the stoic Kanae Kocho—stood on a nearby roof, looking down into the courtyard where Ren was working.
"He looks like a doll," Kanae whispered, smiling gently. "So small and focused."
"HE LACKS SPIRIT!" Shinjuro boomed, crossing his arms. "BUT HIS TOOLS... THEY STRIKE WITH THE PRECISION OF A BLADE. TRULY FLAMBOYANT POTENTIAL!"
Tengen narrowed his eyes. "Let's go talk to the kid. If he can make my bombs more explosive, he's a genius in my book."
They jumped down.
Ren was currently calibrating a Pneumatic Bone-Setter. He was in his "Logic Zone," muttering about PSI and structural integrity.
Crunch.
Ren froze. He didn't turn around. His internal "Social Radar" was screaming. He felt three massive, overwhelming presences behind him. One felt like a furnace (Shinjuro), one felt like a thunderstorm of bells (Tengen), and one felt like a calm, sharp spring breeze (Kanae).
"YO, KID!" Tengen started, reaching out a hand.
Ren's back stiffened. His hair practically stood on end like a cat's. He slowly turned his head. He saw Shinjuro's wild hair and Tengen's massive, muscular frame.
Ren's logical brain calculated the interaction:
Subject A: Too loud.
Subject B: Too sparkly.
Subject C: Too much physical contact imminent.
Probability of survival in a conversation: 0.02%.
Ren's face turned a translucent shade of pink. He didn't even speak. He just dropped his tools, curled into a ball on the porch, and pulled his sleeves over his head, trembling like a leaf.
"...Did we break him?" Tengen asked, his hand hovering in mid-air.
Kanae sighed, pulling the two men back. "He's terrified of us. His heart rate is triple the normal speed. If we stay, he might actually die of embarrassment. Let's leave him be. The Master said to let him work in peace as long as he produces results."
The Hashira vanished back into the shadows. Ren didn't move for twenty minutes.
TP Spent: 2,000 (Remaining: 4,000)
Once he was sure the "Giants" were gone, Ren scurried into his room and locked the door. He entered the Lab, fueled by a desperate need to never be defenseless (or talked to) again.
"If they're going to watch me," Ren muttered, his eyes glinting with a cold, manic light, "I need to make sure they can't get close enough to ruffle my hair."
He spent the next hour working on Nichirin-Alloy Construction.
The Material: He didn't have enough ore for a sword, but he had enough for Micro-Needles.
The Delivery: He modified his wrist launcher. It was no longer just for medical sealant.
The Result: A High-Pressure Needle Gun disguised as a medical bracer. It fires needles coated in a concentrated Wisteria toxin extract, refined in the lab to be 5x more potent than standard mixtures.
He also crafted a Collapsible Periscope.
Logic: If he can see people coming from 100 meters away, he has time to hide in a closet.
That night, Ren didn't go to sleep. He looked at his map of Tokyo. His old home was in the city—a place where demons still hid in the red-light districts and dark alleys.
He didn't want revenge. Revenge was an emotional response. He wanted Data. He wanted to see how a demon's cells reacted to his refined toxins so he could optimize his medical kits.
"I'll go out for thirty minutes," Ren told himself, strapping on his hidden bracer. "Logically, the probability of an encounter is low if I stay in the well-lit areas."
As he climbed out the window, he didn't notice a pair of butterfly-shaped hairclips following him from the trees. Kanae was curious.
Current Status:
TP: 4,000.
Weaponry: Hidden Needle Launcher (Wisteria Poison).
Intelligence: Observed by Kanae Kocho.
Mood: Socially exhausted, scientifically motivated.
Ren stepped into the cool Tokyo night, his small frame swallowed by an oversized haori. To any passerby, he looked like a lost child; to Kanae Kocho, who was tailing him from the rooftops with the grace of a literal butterfly, he looked like a mystery in motion.
"Now, little Ren," Kanae whispered to herself, "what kind of 'walk' requires a hidden wrist-mounted apparatus?"
Ren stopped in a narrow corridor behind a tea house. The scent of decay hit his nostrils—a smell he now knew was the biological signature of a demon.
From the shadows, a low-level demon crawled down the brick wall. It was a bloated thing with multiple eyes, drooling at the sight of a "delicate" child.
"Fresh meat..." the demon hissed. "And such pretty blue hair."
Ren didn't scream. He didn't even flinch. He reached into his sleeve and adjusted a dial on his brass bracer.
'Subject: Low-tier demon,' Ren thought, his brain entering Logic Mode. 'Estimated regeneration speed: Moderate. Distance: 5 meters. Target: Cervical spine.
'
"Logically," Ren said aloud, his voice steady, "you are an inefficient use of biological matter."
The demon lunged.
CLICK-SST!
A silver needle, coated in a shimmering purple liquid, hissed through the air. It struck the demon square in the throat.
The effect was instantaneous. This wasn't standard Wisteria poison; this was Refined Toxin Grade 2, concentrated in the Research Lab. The demon didn't just disintegrate; it liquefied from the inside out, turning into a puddle of purple sludge before it could even finish its leap.
"Data point recorded," Ren muttered, pulling out a small glass vial to scoop up a sample of the sludge. "Toxin-to-mass ratio was slightly overkill. I can reduce the dosage by 12% to save TP."
Caught Red-Handed
"That was... incredibly efficient. And quite terrifying for a nine-year-old."
Ren's heart skipped a beat. That voice. It was like bells—soft, kind, and currently located exactly three feet behind him.
He turned around slowly. Kanae Kocho was standing there, her hands tucked into her sleeves, a gentle but knowing smile on her face.
"I don't recall the Butterfly Mansion teaching 'Projectile Chemical Warfare' to its patients," she said, tilting her head.
Ren's "Cool Researcher" facade shattered like cheap glass.
Face Color: Instant Crimson.
Internal Temperature: Rising.
Logical Escape Routes: Zero.
"I... I was... the atmospheric pressure... the demon was... obstructing the public walkway..." Ren began to stammer, his hands shaking so hard he almost dropped his sample vial.
Kanae stepped closer. She didn't draw her sword. Instead, she reached out and gently took his hand. "Ren-kun, you're shaking. It's okay to be scared after fighting a monster."
"I'm not scared of the monster!" Ren squeaked, his voice cracking. "I'm scared of the... the... proximity!"
He looked at her hand touching his. His brain sent a final distress signal: [CRITICAL SOCIAL OVERLOAD: SYSTEM SHUTDOWN IMMINENT].
Ren didn't even have time to slide down the wall. He just tipped over like a wooden board, fainting right into Kanae's arms.
The Aftermath: The "Butterfly" Apprenticeship
When Ren woke up, he wasn't in his room. He was in the private infirmary of the Butterfly Estate.
Kanae was sitting nearby, examining his wrist launcher with genuine curiosity. "This is fascinating work, Ren. No breathing style, yet you killed a demon in seconds. The Master believes your 'gifts' shouldn't be wasted in a laboratory alone."
She looked at him, her eyes shining with a mischievous kindness.
"From today, you are my personal research assistant. You'll help me develop medicines... and in exchange, I'll make sure the other Hashira don't 'scare' you too much. Deal?"
Ren pulled the blanket over his face until only his ice-blue eyes were visible. He was trapped. He was a genius who wanted to be left alone, and he had just been adopted by the most social, observant woman in the Corps.
"Logically," he whispered from under the blanket, "this is the worst-case scenario."
Current Status:
TP: 4,000.
New Title: The Butterfly's Apprentice.
Bonus: He now has access to the Butterfly Estate's herb garden (Free materials for the Lab!).
Ren sat at a small wooden desk in the Butterfly Estate's apothecary. Shinobu (who is currently a feisty, blunt teenager) was in the corner, aggressively grinding herbs with a mortar and pestle. Every time she looked at Ren, she looked like she wanted to poke him to see if he was real.
'Logically,' Ren thought, sweating slightly, 'if I increase the estate's medical output by 300%, Kanae-sama will have no reason to supervise me. I will be granted the ultimate reward: Being ignored.'
The Research Lab: 2,000 TP Spent (Remaining: 2,000)
Ren waited for his "lunch break" (which he spent hiding in a supply closet) to enter the Lab.
The Problem: Manual herb grinding is slow. Extracting Wisteria toxin via boiling loses 40% of the potency.
The Solution: A Centrifugal Cold-Press Extractor.
The Design: He couldn't bring a motor-powered machine into 1910 Japan without raising eyebrows. He designed a "Clockwork Centrifuge"—it looked like a high-end music box or a complex watch mechanism, but it used gear-reduction to spin at 5,000 RPM via a simple hand-crank.
He also formulated a "Concentrated Healing Salve" that used modern transdermal technology (helping medicine absorb through the skin faster).
The "Bribe" (The Presentation)
Ren approached Kanae later that evening. She was busy reviewing reports of injured slayers. Ren didn't speak; he just slid a small, beautifully crafted brass device and a jar of blue ointment across the table.
"What is this, Ren-kun?" she asked, her voice tilting with curiosity.
Ren took a deep breath, staring intensely at a spot on the floor three inches to the left of her feet.
"The current extraction method is... archaic," he squeaked. "This device uses centrifugal force to separate toxins without heat degradation. It is... 4.2 times more efficient. The ointment will close minor lacerations in half the time."
He paused, his face starting to turn pink. "In exchange... I request... four hours of 'unsupervised research time' per day. No... no check-ins. No head-pats. No... talking."
The Result
Kanae tested the centrifuge. Her eyes widened as a pure, potent purple liquid dripped into the collection vial in seconds—a process that usually took her sisters hours.
She looked at the small, trembling boy who was literally vibrating from the effort of making eye contact.
"You really are a little miracle, aren't you?" She smiled, reaching out—
Ren flinched.
Kanae stopped her hand, laughing softly. "Alright, Ren-kun. A deal is a deal. You've just saved this estate dozens of hours of work. You have your four hours of silence every afternoon. I'll even tell Shinobu to stop 'glaring' at you."
Ren scurried back to his room, his heart finally slowing down.
Success.
With 4 hours of guaranteed privacy and 2,000 TP left, he didn't rest. He opened his System interface. He didn't want to just be a medic. The memory of his family's blood on the tatami mats was a cold, logical weight in his chest.
"Medicine saves the living," Ren whispered, his crystal-blue eyes turning sharp. "But I need to ensure there are fewer people to save."
He looked at the Research Lab's Advanced Metallurgy tab. He couldn't build a gun yet, but he could build "Gravity-Fed Traps" and "Sonic Distraction Decoys."
Suddenly, a notification popped up on his screen:
[New System Mission: The Demon in the Red District]
Objective: Investigate the disappearances in a nearby village.
Reward: 5,000 TP & Blueprint: "Nichirin-Steel Wire."