The Entertainment District was everything Mitsuru had imagined and more.
After entering the city and making his way through the bustling streets, he was immediately struck by the sheer vitality of the place. The roads were packed with people—men and women in elegant kimonos moving between establishments, merchants hawking their wares, the sounds of music drifting from open windows. Lanterns of every color imaginable hung from the beams of houses lining the streets, creating a kaleidoscope of light as evening descended. Red, gold, purple, green—each one advertising a different establishment, each one promising entertainment and escape.
Mitsuru spent the better part of an hour navigating the district, getting his bearings and searching for suitable lodging. Many of the more prominent establishments were clearly brothels or high-end entertainment houses—not appropriate for someone of his current age. Finally, tucked away on a quieter side street, he found a modest inn that seemed respectable enough.
The elderly proprietress looked him up and down with shrewd eyes, taking in the worn katana at his side and his travel-stained clothes.
"Room for how long?" she asked bluntly.
"Three months, possibly longer," Mitsuru replied, pulling out the remaining money from his ticket change. "I can pay weekly if that's easier."
Her eyebrows rose. "Three months? That's quite a stay, young man. What brings you to the Entertainment District for so long?"
"Training," he said simply. "I'm a swordsman. I need time to hone my skills, and this seemed like a good place to... observe various things."
She snorted, but seemed satisfied with the answer. "Training, he says. Well, as long as you pay on time and don't cause trouble, you can stay as long as you like. Room's on the second floor, third door on the left. Meals are extra."
"Thank you."
The room was small but clean—a single futon, a low table, and a small window overlooking the street below. Not luxurious, but Mitsuru had slept in far worse conditions during his previous life. This would do perfectly.
He set down his borrowed katana and sat cross-legged in the center of the room, immediately falling into meditation. His mind was already racing with plans and possibilities.
"Three months," he thought, feeling that familiar thrill of anticipation. "According to the timeline, Tengen and the others should arrive in about three to four months for the mission. That gives me ample time to prepare."
The question was: what could he realistically achieve in that timeframe?
The Demon Slayer Corps had several advanced techniques that separated the elite from the ordinary. The Demon Slayer Mark—the unique pattern that dramatically increased physical abilities and unlock other abilities but came with a price. Transparent World—the ability to perceive everything in slow motion, to see through an opponent's body to their muscles and organs. The Selfless State—eliminating all fighting spirit and becoming impossible to predict.
"The Mark would be better to do with the right conditions," Mitsuru mused, running through what he remembered from the anime. "Life-threatening situations, elevated heart rate and body temperature, the right circumstances... But it's also dangerous. It drastically shortens lifespan, and I have Akane waiting for me back home. I'm not risking that unless I know it will be safe."
He couldn't afford to die young in any world, not when his little sister needed him. He had experiment a bit before trying since while those who unlock the mark are fated to die at 25 yoroichii did live till 90 but he did have that from birth so there's that. Well he also have advantages unique to him so he shelved it for later.
"Transparent World is tied to the Mark in most cases, or requires years of training. Without a teacher who already has it, that's probably out of reach for now."
That left three techniques that were both achievable and valuable: Total Concentration Breathing: Constant, Repetitive Action, and the Selfless State.
Total Concentration Constant was the foundation that separated true Hashira-level fighters from everyone else. Maintaining the breathing technique at all times, even while sleeping, constantly strengthening the body. It was exhausting, required immense dedication, but was absolutely learnable through pure effort.
Repetitive Action was about creating a mental trigger—a memory or thought so vivid that it could push you beyond your limits and achieve maximum concentration in an instant. Different for everyone, deeply personal.
The Selfless State was the hardest for someone like him. It required eliminating all fighting spirit, all bloodlust, all excitement—becoming empty. For someone who lived for the thrill, who felt most alive when facing situation near death, that would be the ultimate challenge.
"Guess than it's decided.," he thought with a smile. Mitsuru opened his eyes, determination settling into his bones like steel.
"Alright then. Three months. Let's see how far I can push myself."
. . . . . .
The first week was spent almost entirely in his room, working on the basics of Total Concentration Breathing.
Sitting cross-legged on the futon, Mitsuru focused on his breath. In through the nose, deep into the diaphragm, expanding his lungs to their absolute limit. Hold. Then out, controlled and steady, maximizing oxygen intake and blood flow.
His chest rose and fell in a precise rhythm. The warm current of energy flowed through his internal organs, circulating with each breath. If anyone else had been in the room, they would have felt something strange—the air itself seemed to move unnaturally, drawn toward the seated figure like he was a black hole consuming everything around him. The decorative painting on the wall swayed gently despite the closed window.
He experimented with different patterns, different intensities. Some made the energy flow faster but more chaotically. Others made it denser but slower. He was searching for his natural rhythm, the breathing pattern that felt right for his body specifically.
Days blurred together. He'd practice breathing for hours until his head swam with oxygen saturation, then break to eat simple meals from street vendors, then return to practice again. The proprietress gave him odd looks when they crossed paths, probably wondering what kind of training involved holing up his room most of the time.
By the end of the first week, he'd found it—his natural breathing rhythm. It was different from what he remembered of the standard patterns. Not quite as aggressive as Flame Breathing, not as flowing as Water Breathing. Something else entirely.
"My own style," he murmured, feeling that thrill of discovery. "Not derived from any existing form."
When he entered this state, his physical capabilities and perception sharpened noticeably. His senses expanded, his reaction time decreased, his muscles responded faster and hit harder. It was intoxicating.
The real challenge began in week two: maintaining Total Concentration Breathing at all times.
Fighting while using it was one thing—adrenaline made that relatively easy. Maintaining it while going about daily activities was harder. Maintaining it while sleeping was nearly impossible.
Mitsuru's days fell into a brutal routine. He'd wake up (after inevitably losing the breathing technique during sleep) and immediately resume it. Then he'd head outside the city to practice his swordsmanship, maintaining the breathing technique through every movement, every form, every repetition.
Parry. Breathe. Slash. Breathe. Thrust. Breathe.
The basic sword techniques he'd learned in his previous life and the kendo forms of this life began to evolve. What had once been rigid, predictable movements became fluid and adaptable. The breathing technique enhanced everything—his speed, his power, his precision.
During the day, he'd purchase simple meals from vendors, always maintaining his breathing. The first time he tried to eat while keeping Total Concentration active, he nearly choked.
Evenings, he'd return to his room and continue meditating, pushing the breathing deeper, making it more natural, more instinctive. He could feel himself getting faster, stronger, more resilient with each passing day.
The nights were the worst. He'd tie a small bell to his wrist before sleeping, and every time the breathing faltered, the movement would wake him. Some nights he woke dozens of times. It was exhausting, maddening, but slowly, incrementally, the periods of maintained breathing grew longer.
By week three, he could maintain it through most of the night. By week four, he rarely lost it at all.
One morning, Mitsuru woke up naturally and realized with a shock that he'd slept through the entire night without the bell waking him once. The breathing had continued unconsciously, his body having fully internalized the pattern.
"Total Concentration: Constant achieved," he whispered, a grin spreading across his face.
With Constant State achieved, Mitsuru turned his attention to Repetitive Action.
This technique was about finding a mental trigger—a memory so vivid that recalling it would instantly push you to maximum concentration and effort. It was deeply personal; everyone's trigger was unique to their own experiences and what mattered most to them.
For Mitsuru, there was only one memory that made sense.
He sat in his room, eyes closed, and let himself remember...
The first time he'd met Akane in this life. The moment his memories had fully integrated and he'd truly understood what he'd been given—a second chance, a sister, someone to protect and cherish.
He had made a promise to himself that day that he would never let her down. Never let anything hurt her. No matter what it cost him, no matter what he had to do, he would protect her.
Mitsuru opened his eyes, that memory crystallizing into a single phrase:
"I made a promise."
Simple. Direct. But loaded with everything that mattered to him.
He tested it immediately, letting the mantra fill his mind while maintaining his breathing technique. Slowly his concentration sharpened to a razor's edge. His perception expanded. Every detail of the room became crystal clear—the grain in the wooden floorboards, the slight draft from the window, the sound of footsteps floors below.
His muscles tensed, ready to explode into action. Energy flooded his system, his heart rate elevated, his body pushed to its absolute peak performance.
"Perfect," he breathed, then let the state relax.
He practiced calling upon it repeatedly—entering that heightened state instantly with just the memory and mantra, then returning to normal. Like a switch he could flip at will.
Over the following weeks, he refined it further. How long could he maintain maximum concentration? How quickly could he enter it under distraction? Could he do it while injured or exhausted?
The training was intense, but the progress was undeniable.
---
The Selfless State was, as he'd predicted, the absolute worst.
The technique required eliminating all fighting spirit, all desire to win, all excitement—becoming empty of intention and emotion during combat. It made you unpredictable because there was nothing for an opponent to read, no killing intent to sense, no tells from your emotional state.
For someone like Mitsuru, who lived for the thrill, who felt most alive when his life was on the line, who got excited at the prospect of fighting strong opponents, it was torture.
His first attempts were laughable. He'd try to empty his mind during sword practice, but the moment he executed an attack, he would get excited. The moment he thought about facing demons or other Hashira, his fighting spirit would flare. The moment danger appeared (even imaginary), his blood would sing with anticipation.
"This is impossible," he muttered after his twentieth failed attempt in a single day. "How am I supposed to fight without any emotion at all? Fighting is inherently exciting!"
But he refused to give up. If Tanjiro can do it, so could he.
He tried different approaches. Meditation techniques. Mental exercises. Nothing worked until he stumbled on a different perspective entirely:
"It's not about having no emotions," he realized one evening, watching the sunset from his window. "It's about separating action from emotion. The body moving without thought."
That was it. Breathing had become so natural, so instinctive, that he didn't feel anything about it anymore. It just happened. What if he could make combat the same way?
He started smaller. Instead of trying to enter the Selfless State during full combat practice, he focused on single movements. One strike, performed with perfect technique but zero emotional investment. Like it was just... something that happened.
Gradually, he could extend it. One strike. Two strikes. A sequence of three movements.
It was exhausting in a completely different way than physical training. Mental discipline on a level he'd never needed before.
By week twelve, as the time was approaching towards the Entertainment District arc, Mitsuru could maintain the Selfless State for approximately five minutes during active combat practice. Not nearly as long as he'd like, but it was a start. More importantly, he could enter it at will now, could flip that switch and become empty even in the middle of a fight.
Five minutes where he became impossible to read, where his attacks had no intent, where an opponent couldn't sense what he'd do next.
"It'll have to be enough," he thought, sheathing his practice sword. "Five minutes can decide a fight."
---
It was the final week of his third month when Mitsuru met them.
He'd been training outside the city during the day. The sun was bright overhead as he worked through his forms, combining Total Concentration Constant with swordsmanship. Each movement sharper, faster, more powerful than three months ago.
He could feel it—he was incomparably stronger now. Not Hashira-level yet, but no longer helpless.
From a distance, he heard rustling sounds. Multiple people approaching.
He turned toward the sound, and saw three boys walking over. A boar-headed person bounded ahead with wild energy. Behind him, a young man with fiery red hair, a wooden box on his back, and a distinctive scar. Last, a young man with golden-yellow hair, face tear-stained, expression between terror and misery.
The characteristics were impossible to mistake. These were the three young Demon Slayers from the anime: Tanjiro, Zenitsu, and Inosuke.
But where was—
He felt a slight vibration behind him. Incredibly fast, perfectly controlled.
Mitsuru didn't move, but spoke clearly: "Isn't it impolite to approach someone like that, sir?"
He turned to find Tengen Uzui standing feet away. Despite expecting it, Mitsuru felt genuine thrill. The sheer skill in that movement, the control...
Up close, Tengen was even more imposing—tall, powerfully built, distinctive white hair and flashy headband. Despite his relaxed stance, every muscle was coiled and ready.
But what really caught Mitsuru's attention was how Tengen's eyes had narrowed slightly, the subtle shift in posture. Not hostile, but... curious.
Tengen's expression shifted to genuine interest. He laughed heartily and clapped Mitsuru's shoulder with several dull thuds.
"Well, well! You sensed me, " His grin widened. "You're about half as flamboyant as I am!"
Mitsuru raised an eyebrow. "Is that... good?"
"It's fantastic! Most people don't get above a quarter!" Tengen's eyes gleamed with interest. He struck a dramatic pose. "I am Tengen Uzui, the God of Festivals and Flamboyance! What's your name, sword guy?"
Despite knowing about Tengen's personality from the anime, seeing it in person still made Mitsuru suppress a smile. The man was genuinely this extra, but underneath was a razor-sharp mind evaluating every detail.
The three younger Demon Slayers caught up. Zenitsu looked mortified and elbowed Tanjiro.
"He's making us look bad," Zenitsu whispered urgently, though not quietly enough. "I don't know him. We've never met."
Tanjiro looked confused, his earnest face showing genuine bewilderment. "Eh? But we've been traveling together for—"
"HAHAHAHA! I am the King of the Mountain—Inosuke!" Inosuke suddenly shouted, leaping forward and striking a dramatic pose. "Fight me!"
Tengen's expression froze mid-smile. He looked at Inosuke, then at Mitsuru, then back to Inosuke.
"...I don't know him either," Tengen said flatly.
Mitsuru couldn't help it—he laughed. These people were exactly as chaotic as he'd remembered, maybe even more so in person.
"My name is Mitsuru," he said, composing himself and offering a slight bow, though internally he was already itching to spar with at least one of them. "I'm a wandering swordsman. I couldn't help noticing you all moving through the area—each with very distinctive styles."
'A wandering swordsman with Constant mastered at his age? That's quite a story. Maybe I should bring him with me after the mission.'
Tengen's playful expression took on a sharper edge, but more curious than suspicious. "A wandering swordsman, you say? You are quite good for one? Where'd you train?"
Before Mitsuru could respond, Tanjiro stepped forward with characteristic openness, his genuine smile completely cutting through any tension.
"Hello! My name is Tanjiro Kamado," he said warmly. "That's Zenitsu Agatsuma, and the enthusiastic one is Inosuke Hashibira. We're heading to the Entertainment District for a mission!"
Mitsuru felt something twist in his chest. That naturally kind spirit. This kid had lost his entire family, seen horrors that would break adults, yet still approached strangers with openness and trust.
Given the chance, he'd personally drag Muzan into sunlight and watch him burn.
"It's good to meet you, Tanjiro," Mitsuru said warmly. "And all of you." He glanced at Tengen. "If you don't mind, I could guide you to the Entertainment District. I've been staying there for a few months now, so I know the area fairly well."
"You've been there for months?" Tengen's interest clearly piqued. "Doing what, besides training?"
"Observing, mostly. Listening to rumors. The Entertainment District has... interesting patterns, if you know what to look for."
Tanjiro's eyes lit up. "That would be wonderful! Thank you!"
But he immediately looked to Tengen for approval, clearly respecting the Hashira's authority.
Tengen studied Mitsuru for a long moment, those sharp eyes taking in every detail. The worn but well-maintained sword. The controlled breathing pattern marking Constant mastery. The calm demeanor despite being confronted by four Demon Slayers. The fact he'd sensed Tengen's approach.
Finally, Tengen's grin returned, wider than before.
"You know what? I like you, sword guy. You've got style!" He gestured broadly then his expression turned more serious. "But fair warning—if you're hiding something dangerous, these ears will catch it. I can hear a heartbeat change from a hundred yards away."
"Noted," Mitsuru said with a slight smile. "Though I assure you, I'm just a swordsman trying to get stronger."
"Aren't we all!" Tengen laughed and clapped his shoulder again. "Come on then, let's head to the Entertainment District! The lights, the music, the beautiful people—it's paradise!"
"Uzui-san, we're supposed to be on a mission," Zenitsu said nervously. "Finding your wives, remember?"
"It can be both, Zenitsu! That's what makes me a genius!"
As the group began walking, Mitsuru fell into step beside them. Inosuke kept bounding around, challenging him to fight.
"Fight me! You look strong!"
"Maybe later," Mitsuru said, and meant it—the prospect was genuinely appealing. "Maybe after you finish your mission."
"Promise?!"
"Promise."
Zenitsu alternated between nervous chatter about demons and desperate questions. "Are there really pretty ladies there? What if a demon eats me before I can get married?!"
Tanjiro asked genuine, focused questions about the district layout, clearly mentally mapping for the mission.
And Tengen, despite his flamboyant attitude, kept Mitsuru in peripheral awareness the entire time. Not hostile, but intensely curious. Occasionally making comments or questions, clearly testing reactions.
Just like that four demon slayer and a dimensional traveler entered Upper moon 6's playground..