The air smelled of iron and expensive incense.
Ren's eyes fluttered open. His head felt like someone had used his skull as a forge for a Nichirin blade. He stared up at the charred rafters of a ceiling that looked way too expensive for his tax bracket.
"Who am I...?" he croaked, his voice sounding like sandpaper on glass.
He blinked. Wait. That's the first question. I'm supposed to ask three.
"Where am I?" He looked at his hands. Small. Pale. Not the hands of a man who spent his life behind a computer screen. "And... what the hell am I?"
[System Initializing...]
[Memory Synchronization: 10%... 40%... 100%]
The floodgates opened. Images of a lavish life in Tokyo—tutors, silk kimonos, and a father who smelled like old books—slammed into his brain. Then came the red. The screaming. The sound of something feasting in the dining room.
He remembered hiding in the floorboards. He remembered a man with a sword and a patterned haori pulling him out of the wreckage as the sun began to rise.
"Ren Aoki," he whispered. "I'm a nine-year-old orphan in Taisho-era Tokyo. Great. Fantastic start. 10/10 would not recommend."
As he sat up, a translucent screen flickered in his peripheral vision. No maps, no "Auto-Battle" buttons, just a cold, industrial interface.
[Current TP: 10,000]
[Warning: Research Lab Access requires 1,000 TP per 10 minutes.]
"Wait, I only have one shot at this?" Ren's logical brain kicked into overdrive. If a demon slayer saved him, they were likely still nearby. He needed a weapon now before he was hauled off to an orphanage or, worse, a training camp where he'd have to actually exercise.
He closed his eyes and entered the System Research Lab.
The world shifted. Suddenly, he was standing in a high-tech workshop filled with lathes, 3D printers, and chemical vats.
[Timer: 09:59... 09:58...]
"Ten minutes?!" Ren shrieked, his "cool genius" persona evaporating into pure panic. "That's not enough time to build a toaster, let alone a gun!"
He scrambled to the assembly table. His hands moved with a mechanical instinct he didn't know he had. He didn't have time for a modern rifle. He needed a Derringer—small, simple, and deadly.
Clank! Whirr! Sizzle!
He spent 800 TP on materials and the final 200 TP on a "Special Item" from the shop: Nichirin Ore Dust.
Now he have only 8,000 TP left .
With 30 seconds left, he shoved the dust into the lead casting mold. Two .41 caliber bullets dropped into his hand, shimmering with a faint, sunlight-trapping hue.
[00:01...]
[Expelling User from Lab. Thank you for your patronage.]
Ren "spawned" back into the real world, sitting on the ruins of his porch, clutching a snub-nosed, steam-finished Derringer.
"Okay," he muttered, regaining his composure. "Logically, I am now the most dangerous nine-year-old in Japan. I just need to—"
"Oh! You're awake!"
Ren jumped three feet in the air. A young Demon Slayer—a "Kakushi" assistant—was standing right there with a tray of rice porridge, looking at him with immense pity.
"You poor thing... losing your whole family at such a young age," the assistant said, her eyes tearing up. She reached out and patted Ren's frost-blue hair. "Don't worry, little Ren. We'll take care of you."
Ren froze. His face went from pale white to a shade of red that rivaled a sunset. His logical brain started throwing '404 Error' messages.
"I... uh... the trajectory of... kindness... is..."
Steam literally hissed from his ears.
THUD.
Ren Aoki, the multiversal genius architect, fainted face-first into the dirt from pure social embarrassment.
Ren spent the next three days in a state of "controlled recovery." He was currently staying at a temporary estate owned by a wealthy branch of the Ubuyashiki family (the sponsors of the Demon Slayers). They assumed he was just a shell-shocked orphan, but behind his vacant, crystal-blue stare, his brain was a factory.
Ren sat on the engawa (porch), writing in a notebook. He had analyzed his situation:
Civilian Status: He cannot carry weapons.
Physicality: He has the stamina of a wet paper towel.
The System: It doesn't give him "Gamer Sense." It only provides the Lab and TP.
"If I can't kill them yet," Ren whispered, his logic-driven mind clicking into place, "I'll make sure the people who can kill them don't die."
Ren waited until nightfall. He pulled the covers over his head and entered the System Research Lab.
[TP: 8,000]
[Timer: 10:00... 09:59...]
He didn't go for the assembly line this time. He went to the Chemical Lab and Precision Milling station. He had brought a small pouch of Wisteria flowers he'd swiped from the estate gardens.
Project: Advanced Tactical First Aid Kit (Taisho Edition)
Item 1: Hemostatic Quick-Clot Gauze. (Using lab tech to treat silk fibers with refined minerals to stop arterial bleeding in seconds).
Item 2: Wisteria-Infused Adrenaline. (A refined stimulant to keep a slayer moving even with broken ribs).
Item 3: The "Aoki" Pressurized Injector. A brass-and-glass device that looks like a Taisho-era antique but functions like a modern EpiPen.
He worked like a demon. His hands were a blur of motion, mixing compounds and lathing brass casings.
[00:15... 00:10...]
He grabbed the finished kit—a sleek, leather-bound box that looked like a doctor's bag—and the lab vanished.
The next morning, the Kakushi assistant, Aoi (who is much younger here, perhaps an apprentice), came in to check on Ren. She found him sitting perfectly upright, surrounded by complex diagrams of human anatomy and chemical formulas drawn in charcoal on the floor.
"Ren-kun... what is all this?" she asked, worried.
Ren didn't look up. "I have calculated that the survival rate of the average Demon Slayer is abysmal due to blood loss. I have created a solution."
"A solution? You're nine! You should be eating your soup!" She tried to take his notebook away.
Ren's face went instantly bright red. He clutched the notebook to his chest and began to stammer. "T-the molecular structure... it's... you're standing too close. The oxygen-to-kindness ratio is... suboptimal..."
He started to sway.
"Ren-kun? Are you okay?" Aoi reached out to touch his forehead.
"Critical... social... failure..." THUD. Ren Aoki, the boy who just invented 21st-century medical tech in 1910, fainted because a girl spoke to him too loudly.
The Aoki estate was gone, but the Butterfly Mansion (or a similar medical ward used at the time) was very real. Ren had been moved there under the supervision of the Corps' doctors. To them, he was a tragic genius child who spent all day staring at herbs.
To Ren, he was just waiting for the timer to start.
"Logically," Ren muttered, sitting in his room, "a sword is a primitive tool. It relies on the user not having their internal organs turned into mush. If I can't give them better armor yet, I'll give them a better 'Undo' button for their injuries."
Ren entered the Lab. With 6,000 TP remaining and a full hour on the clock, he didn't panic. He moved with the grace of a master clockmaker.
The Goal: A "Compact Surgical Suite."
The Innovation: In the Taisho era, surgery was slow and infection was a death sentence. Ren used the machinery to create Polymer-based Sutures (synthetic threads that don't rot) and a Centrifuge powered by a hand-crank to separate Wisteria toxins from healing agents.
He also crafted a concealed wrist-launcher. It wasn't a gun—it was a spring-loaded tool that could fire a "Cauterizing Bolt" at a wound from a distance. If he was going to be around slayers, he didn't want to get close to the blood. Or the people. Especially the people.
Ren was jolted back to reality by the sound of heavy footsteps and the smell of copper. A Slayer was carried in, his haori shredded, a deep gash across his thigh from a Lower Moon's minion. The local medic was frantic, trying to apply pressure with simple cloth.
"The artery is nicked! He won't make it to the capital!" the medic cried.
Ren walked over, his face a mask of cold, logical indifference. He reached into his oversized sleeve and pulled out a small, silver canister—his Aoki-Brand Aerosol Sealant.
"Move," Ren said. His voice was small, but it had the authority of a Chief Surgeon.
"Kid, get out of here! This isn't a place for—"
Ren didn't wait. He stepped forward and sprayed the wound. A localized chemical reaction occurred—the foam expanded, hardened, and instantly sealed the tear in the flesh. The bleeding stopped. The Slayer's heart rate stabilized.
The room went silent. The medic stared at Ren. The wounded Slayer stared at Ren.
The wounded Slayer, a boisterous man with a scarred face, reached out a shaky, giant hand and ruffled Ren's frost-blue hair.
"Kid... I don't know what magic that was... but you're a little hero, aren't you? You saved my life. Thank you." He gave Ren a weak, but genuine, thumb-up and a toothy grin.
[INTERNAL SYSTEM ERROR: UNEXPECTED AFFECTION DETECTED]
Ren's brain halted.
Step 1: Receive gratitude. * Step 2: ...Analyze? * Step 3: ABORT MISSION.
Ren's pale skin turned a violent shade of magenta. His crystal-blue eyes began to spin in circles. He tried to say something logical like "It was merely a chemical necessity," but all that came out was a high-pitched "Meep."
"He's blushing! How cute!" a nurse giggled from the doorway.
That was the final straw. Ren's knees buckled. He didn't just faint; he did a slow, dramatic slide down the wall until he was a pile of blue hair and red cheeks on the floor.