The one-year mark had arrived.
For Aris Thorne—now fully and irrevocably Leorio Paradinight—the last twelve months had been an exercise in absolute biological discipline. He had grown. The 185 cm frame he had inhabited upon arrival had stretched and filled out, reaching a towering 193 cm. His muscles were no longer just the result of a high-protein diet; they were dense, corded ropes of functional power, the physique of a man who spent his days hauling heavy game through wilderness and his nights rewriting the laws of his own anatomy.
He had abandoned the spiky, unruly hair of his predecessor. Now, his coal-black locks were long enough to be pulled back into a sharp, pragmatic top-knot—the "man-bun" style reminiscent of a certain Jaeger from another world. It suited him. It gave him the air of a wandering warrior-monk rather than a rowdy teenager.
He sat atop the highest peak of the Yorbian range, overlooking the clouds. He had hiked here to escape the "noise" of humanity.
Internal Monologue: Twelve months. 365 days of knocking on a door that has no handle. I haven't read a single book on Nen as I couldn't find one. I tried to see if a mind trained in the pinnacle of Earth's science could deduce the energy of the soul through pure anatomical focus. If the brain generates an electromagnetic field, then the 'aura' must be the macro-extension of that field.
Leorio closed his eyes. He didn't visualize a "warm light" or "magical steam." He visualized his nervous system as a grand conductor. He focused on the synapse—the tiny gap between neurons. He imagined the collective electrical discharge of billions of neurons firing in a single, synchronized pulse.
Synchronize. Don't just fire; resonate.
Suddenly, something inside his solar plexus snapped. It wasn't painful. It felt like a dam of ice-cold water had suddenly been hit by a thermal vent. A torrential flood of energy erupted from his center, rushing through his limbs with the force of a hurricane.
"There it is," he gasped, his eyes flying open.
Visible to the naked eye, a shimmering, translucent shroud of white energy began to leak from his pores. It was chaotic. It was violent. It was his life force bleeding away into the atmosphere.
Internal Monologue: The nodes are open. If I don't close them or contain this, I'll die of exhaustion in an hour. This is the 'leak' I've theorized about. I need to—
Before he could even attempt to exert his will, a crystalline chime echoed directly in his auditory cortex. It wasn't a sound from the outside world; it was a digital resonance.
[CONGRATULATIONS TO HOST FOR MEETING THE AWAKENING CONDITIONS.]
Leorio froze. "A system? Now?"
[NEN ABILITY RECEIVED: GATE OF OTHERWORLDS (SPECIALIST TYPE)]
[AS A RESULT OF NATURAL AWAKENING, YOU HAVE RECEIVED A SPECIAL AWARD: MASTERY OF THE FOUR BASIC PRINCIPLES.]
Suddenly, the chaotic aura surrounding Leorio stilled. Without him lifting a finger, the energy pulled tight against his skin, forming a perfect, shimmering layer of Ten. The leakage stopped. The exhaustion vanished, replaced by a feeling of absolute solidity. Then, the aura flared with a sudden intensity (Ren), vanished completely into his body (Zetsu), and finally projected outward in a controlled burst (Hatsu).
He felt the knowledge settle into his brain like a pre-installed program. He knew the feeling of these states as if he had practiced them for twenty years.
"Skimming the grind," Leorio muttered, a sharp, satisfied smirk crossing his face. "I spent a year trying to find the door, and the 'System' just gave me the keys to the entire building. Efficient. I like it."
[DO YOU WISH TO KNOW YOUR STATUS?]
"Yes," he said aloud, his voice echoing off the mountain peaks.
Status Window
Host Name: Leorio Paradinight
Age: 18
Height: 193 cm
Nen Type: Specialist
Primary Ability: Gate of Otherworlds
Worlds Available: Demon Slayer (Kimetsu no Yaiba)
Mastery: Ten, Zetsu, Ren, Hatsu (Mastery Level)
Leorio focused on the text for Gate of Otherworlds. Detailed information flooded his mind, laid out with the cold clarity of a surgical report.
Ability: Gate of Otherworlds
Function: Allows the user to transition their physical body into a different dimensional plane (Otherworld).
Condition (The Vow): To activate the gate, the user must sacrifice 99% of their current Nen reserves.
Penalty: Upon returning to the "Hunter" world, the user is forced into a state of Zetsu for 24 hours.
Cooldown: Can only be used once every 30 days.
Constraint: New worlds are unlocked only once every 12 months.
Compatibility: While in an Otherworld, the user can only utilize the specific power systems of that world and their base Nen. They cannot use powers gained from World A while in World B. However, all physical gains and skills are permanent.
Leorio stood up, his tall frame casting a long shadow.
Internal Monologue: Demon Slayer. A world of breathing techniques, swordsmanship, and biological regeneration through demons. My physical power right now is already monstrous—I can probably crush a demon's skull with my bare hands—but my technique is non-existent. I'm a surgeon who lost his scalpel and started using a sledgehammer. I need the sword.
The Hunter Exam was exactly one year away. One year in this world, or one year in another? The system didn't specify time dilation, but even if it was 1:1, a year of dedicated swordsmanship training under the masters of the Sengoku or Taisho eras would turn him into a nightmare for his future opponents.
"One year to become a master," Leorio said, his eyes gleaming with a cold, predatory light. "System, activate Gate of Otherworlds: Demon Slayer."
The 99% drain was instantaneous. He felt his massive reservoir of aura vanish, leaving him feeling dangerously light, almost fragile. A swirling vortex of violet energy tore open in the air before him—a literal wound in reality.
He didn't hesitate. He stepped through.
The first thing Leorio noticed was the snow.
It wasn't the dry, high-altitude snow of the Yorbian mountains. It was a wet, heavy, biting cold that smelled of charcoal and pine. He was standing on a mountain path, his 193 cm frame towering over the small, stunted trees of the Japanese wilderness.
He adjusted his top-knot and looked down at his clothes. He still wore his sturdy Saherta traveling gear, looking entirely out of place in the Taisho-era landscape.
Suddenly, the sound of steel clashing and the scent of blood hit his nostrils.
Distance: 200 meters. Vector: North-Northwest.
Leorio moved. Even with 1% of his Nen, his physical conditioning was far beyond any normal human. He cleared the distance in seconds, leaping over snow-covered logs with the grace of a panther.
He broke into a clearing and skidded to a halt.
The scene was iconic. A boy with a checkered haori—Tanjiro—was desperately shielding a girl with a bamboo muzzle in her mouth. Standing opposite them was a man in a half-and-half patterned haori, his blade poised with a terrifying, stoic lethality.
The Hashira's eyes flickered toward the newcomer. He didn't lower his sword, but his brow furrowed. Leorio was a giant—nearly a foot taller than Giyu—and he radiated a raw, physical pressure that felt like a mountain standing on two legs.
"Who are you?" Giyu asked, his voice like ice. "You are strong ,you aren't a civilian."
Leorio stepped forward, raising his large, calloused hands to show he was unarmed. He looked at Tanjiro, then back to Giyu. He knew this moment. This was the crossroads.
"My name is Leorio," he said, his Japanese coming out fluent thanks to the System's auto-translation. "I'm a traveler... and a student of medicine. I've been looking for a way to protect people, but I lack the skill. I've walked a long way to find a master who can teach me the 'Breath' that can sever a nightmare."
Giyu stared at him. He looked at Leorio's hands—the hands of a man who had killed beasts and saved lives. He saw the "Eren-style" bun, the hardened gaze of a man who had already died once, and the sheer, overwhelming physical potential of the tall man.
Giyu said flatly. "Your center of gravity is high. You would be an easy target for a Lower Moon."
Leorio smiled—a sharp, confident grin that didn't reach his tired eyes. "Then teach me how to move so that I becomes their funeral. I have the strength. I have the will. All I need is the path."
Giyu looked at Tanjiro, who was shivering in the snow, then back to the giant. The Water Hashira sensed something strange about Leorio. The man had almost no 'presence' (due to the 99% Nen drain putting him in a near-Zetsu state), yet his muscles were taut like drawn bowstrings.
"I am sending this boy to a man named Sakonji Urokodaki at the foot of Mount Sagiri," Giyu said, sheathing his sword in a blur of motion. "If you can keep up with him, and if you can survive the mountain, perhaps you will find what you seek."
"I can keep up," Leorio said.
He walked over to Tanjiro, who was staring up at him in awe. Leorio reached out a hand—a hand that could crush a skull but was currently as gentle as a surgeon's—and helped the boy up.
"Let's go, kid," Leorio said softly. "We've both got people to save, and a lot of work to do."
The journey to Mount Sagiri was a revelation.
Internal Monologue: My Nen is almost gone, but the physical strength I built in the Hunter world is still here. In this world, I am a freak of nature. My bone density, my lung capacity... I'm already at the level of a Hashira in terms of raw power. But Giyu was right. I'm clumsy. I'm used to using my size to bully opponents. Against a demon with Blood Demon Arts, I'd be diced before I could land a punch.
As they ran, Leorio watched Tanjiro. The boy was struggling, his breath coming in ragged gasps.
"Tanjiro," Leorio called out, his voice steady even as he ran at a pace that would kill a marathon runner. "Don't breathe from your throat. Breathe from your lower abdomen. Expand the diaphragm. You're a coal burner; you know how to stoke a fire. Treat your lungs like a furnace."
Tanjiro looked up, startled. "L-Leorio-san? How do you know that?"
"I'm a doctor," Leorio said. "I know how the bellows work. If you don't control your oxygen, you'll never reach the mountain."
For the next few days, Leorio became a mentor before he even became a student. He used his knowledge of neurobiology and kinesiology to correct Tanjiro's gait, helping the boy conserve energy.
When they finally reached the cabin of Sakonji Urokodaki, the man in the tengu mask was already waiting.
Urokodaki didn't speak at first. He looked at Tanjiro, then his gaze drifted up... and up... to the 193 cm man standing behind him.
"This one," Urokodaki said, his voice muffled by the mask. "He is not a normal boy."
"I'm a student," Leorio said, bowing deeply—a gesture of respect he had learned was vital in this culture. "I wish to learn the Water Breathing. I wish to learn the sword."
"You are too old," Urokodaki said. "And your body is already set in its ways. You are built for a hammer, not a needle."
"Then teach me to use a sword as if it were a scalpel," Leorio replied, rising from his bow. "I've spent a lifetime learning how to navigate the human brain. I think I can handle a piece of steel."
Although He didn't knew what a scalpel was Urokodaki huffed. "The mountain will decide that. Both of you. Follow me."
The next several months were a descent into a specific kind of hell.
While Tanjiro struggled with the traps and the thin air, Leorio faced a different challenge. The traps that would bruise Tanjiro would shatter against Leorio's conditioned skin, but Urokodaki adjusted accordingly. For Leorio, the traps were heavier, faster, and aimed at his joints.
Internal Monologue: This is perfect. Urokodaki isn't just training my body; he's training my perception. In the Hunter world, I was training for power. Here, I'm training for 'Flow.' Water Breathing is about the absence of rigidity. I have to forget I'm a 193 cm tank and remember I'm 70% water.
Leorio stood in front of a massive waterfall, his eyes closed. He was shirtless, his muscular torso gleaming under the freezing spray. He wasn't using Nen. He was practicing the Total Concentration Breathing.
Inhale. Expand the capillaries. Force the oxygen into the red blood cells. Increase the heart rate without increasing the blood pressure.
"Total Concentration..." Leorio whispered.
He felt his heart hammer against his ribs like a war drum. His surgeon's mind mapped the oxygen flow. He could feel the gas exchange in his alveoli.
...Constant!
A surge of heat erupted in his chest. It wasn't Nen. It was the "Life Force" of this world—the sheer biological optimization of the human frame.
He drew a practice wooden sword and swung.
First Form: Water Surface Slash!
The strike was blindingly fast. It didn't have the "magical" water effects yet, but the sheer displacement of air created a whistling sound that cut through the roar of the waterfall.
"Not bad," Urokodaki's voice came from the shadows. "But you are still too stiff. You are trying to 'operate' on the air. Stop being a doctor. Be the river."
Leorio wiped the water from his eyes, his long hair clinging to his neck. He looked at his hands. They were steady.
Internal Monologue: One year. I have one year in this world to master the breath and the blade. When I return to the Hunter Exam, I won't just be 'The Tall Guy with the Suit.' I will be a Specialist who can breathe life into a blade.
He looked toward the horizon, where the sun was beginning to rise over Mount Sagiri.
"Tanjiro is progressing well," Leorio thought, watching the younger boy practice his swings in the distance. "And I'm getting there. By the time I meet Hisoka... I won't need a wooden pole. I'll have the Breath of the God of Medicine."
He turned back to the waterfall and raised his sword again.
He had a world to conquer, and he was only just getting started.