"Interesting…"
Ryuji watched Nezuko's body continuing to evolve. While Tanjiro kept rhythmically working through his Sun Breathing forms, Ryuji casually slipped two fingers into Nezuko's mouth.
Life force and spiritual energy flowed once more into her. Nezuko's soft lips and warm, pink tongue instinctively pressed and curled around his fingers, the gentle, velvety sensation almost amusing to Ryuji.
Tanjiro, however, wasn't quite as entertained.
When he saw Ryuji's fingers in his little sister's mouth—while she was actively sucking on them—his face, already flushed red from forcing a breathing style ill-suited to him, went crimson. Steam all but seemed to rise from his head.
He froze mid-form, nearly choking, and pointed a trembling finger toward Ryuji.
"R–Ryuji-sama! What on earth are you doing to my sister?!"
Propping his chin up with one hand, Ryuji raised the other—Nezuko still clamped down tight, hanging from his fingers like a hooked fish.
"As you can see, I'm feeding your sister. I can channel life force directly, and she's undergoing a transformation I don't fully understand myself. Better she feeds on me than… on people, right?"
The words—and the sight of Nezuko stubbornly biting down on Ryuji's fingers—left Tanjiro momentarily speechless, his mind unable to process what he was seeing.
But Ryuji had noticed something curious.
Along with his life force, Nezuko's body seemed to be absorbing… something else. He couldn't say exactly what, but he could feel it: she was drawing in a substance from the air around them. The longer she stayed close to him, the more he sensed something gathering here.
Then, his Geiger counter crackled to life.
He'd kept it with him ever since suspecting that the "stellar" demon he fought might have been emitting radiation. He just hadn't expected it to go off now.
Startled, he glanced down at Nezuko—only to see a faint luminescence slowly enveloping her form. Looking up, he realized the stars overhead were shining noticeably brighter than before.
"…Huh?"
He immediately held Nezuko at arm's length. The Geiger counter quieted, and when he looked back at the night sky, the brightness had dimmed to normal. Pulling her close again made the counter resume its rapid clicks—and the starlight flared once more.
"Well, that's… something."
The first thought that came to mind was "stellar spirit energy," but the relentless ticking told him otherwise. No—she was absorbing stellar radiation.
"Radiation is still energy… which means…"
He turned to Tanjiro, who was still looking confused.
"Tanjiro—your sister might not fear sunlight much longer."
Tanjiro blinked at him in disbelief, so Ryuji pointed upward.
"She's drawing power from the stars above. Many of them are suns just like ours—too far away to light up our world, but still radiating power."
It was the simplest explanation Ryuji could give, though Tanjiro clearly wasn't buying it.
Just then, Nezuko slipped free from Ryuji's fingers, falling like a fish that had shaken off the hook. Before she hit the ground, her body shrank rapidly, reverting to the size of an eight-year-old doll.
"Nezuko!"
Tanjiro dove forward, catching her and quickly tucking her into the newly rebuilt box he carried—making sure she'd be safe come dawn. Ryuji's talk of her overcoming the sun sounded wonderful, but it was far too unbelievable for him to gamble on.
"Fine, believe what you want," Ryuji said with a shrug. "But at this rate, she might become the ultimate lifeform."
He could still feel the changes within her, but there was no point arguing. People rarely accepted the truth until it stood in front of them.
Instead, he turned the conversation elsewhere.
"By the way, Tanjiro—your breathing has several points that don't suit you. For example, in this move, you should skip one inhale. Your lungs can't handle the pressure buildup—push it too far, and you'll tear lung tissue, end up with pneumonia or even emphysema."
Ryuji demonstrated a stance where the lungs would be under extreme compression, rapidly exchanging oxygen but at the cost of severe strain.
"Ah? Oh…"
Tanjiro hesitated, then followed the stance with his wooden sword, matching Ryuji's suggested breathing rhythm. Sure enough, his body felt lighter, and his swings gained a noticeable burst of strength.
"And this move—change the rhythm. As it is, your breathing can't keep up, and you'll damage yourself."
Ryuji spoke again, making two precise gestures as he did so.
"Alright."
Tanjiro shifted into the new stance without hesitation, though he couldn't quite understand why Ryuji was being so serious. To him, this was nothing more than a family dance passed down through generations.
"Um… Ryuji-sama, this is just a dance. Do we really need to treat it so seriously?"
Ryuji froze, looking at him as if he'd just said the most ridiculous thing imaginable. His gaze sharpened.
"…Are you serious?"
"Ah?" Tanjiro blinked in confusion, not grasping what he meant.
Ryuji sighed inwardly. No wonder they call him the uneducated … his brain really does run on a delay.
"Do it the way I just told you—like you're using a breathing technique. Swing the blade as if you're channeling your breath through it."
The moment Ryuji's words landed, the obedient boy nodded without even asking why.
"Mm!"
Tanjiro swung his sword. Halfway through the arc, flames like the tongues of a solar flare burst to life along the blade. With a single slash, he split the stone before him cleanly in two. But that wasn't all—those searing flames burned so hot that they melted everything along the sword's path.
Tanjiro stared at his weapon in disbelief. The sheer power of that strike was worlds apart from his Water Breathing—like they belonged to entirely different dimensions.
"Th–this is…"
His mouth hung open, his wide eyes turning to Ryuji as if asking whether this miracle was even real.
"You haven't realized it yet? That family dance of yours is actually an incredibly powerful breathing style. The problem is, while it's strong, it puts a huge strain on the body."
Ryuji crossed his arms, studying the faint marks left on Tanjiro's skin. Even he had to admit he was impressed.
This world's breathing techniques were already strange enough to produce flashy effects with sword swings, but in a land where demons had supernatural abilities and even yokai walked the earth, that wasn't the strangest part.
What was surprising was the sheer destructive force of Sun Breathing. Its area and intensity didn't quite match his own dragonfire, but considering this was Tanjiro—someone ill-suited to the style—the result was staggering. If Yoriichi Tsugikuni himself used it… well, that would be an entirely different league.
The truth was, the strength of Sun Breathing came not from the technique itself, but from Yoriichi. The style wrung every ounce of potential from the human body—most people would burn out in no time.
If Tanjiro pushed himself to use the technique seriously dozens of times, he'd be dead. To unleash its full power… he might not even last that long. His body simply couldn't take it.
"This breathing style puts enormous pressure on your lungs. If you don't use the modified version I taught you, a few uses will leave you sick, and a few dozen will kill you outright."
Ryuji knew its potential well. In his own hands, paired with dragonfire, it could probably wipe out every living thing along an entire street. Not the buildings—steel and concrete would survive—but life itself would be gone.
Still, Sun Breathing was a double-edged sword, much like his own Blood Qi Arts. Breathing styles altered blood circulation through controlled respiration, boosting physical power. Blood Qi Arts, on the other hand, directly changed the blood itself, even reshaping bodily functions—two very different systems.
In pure amplification and growth potential, Blood Qi Arts were stronger. Breathing styles were simply easier to learn. Without a demon's resilience, training in Blood Qi Arts was practically a death sentence.
That was why Ryuji could dissect and improve breathing styles so easily—master one, understand the principles, and you could reverse-engineer the rest.
Still, it wasn't without gain. He'd learned a few unique ways to stimulate the body—details Blood Qi Arts ignored—that could be used in certain situations.
"So, if you're really set on using this style, you'd better stick to my version and then refine it yourself."
He gave the warning while eyeing Tanjiro's frail frame. The modified Sun Breathing he'd taught had only about half the power, but its damage to the body was within tolerable limits. Of course, demon slayers were impossible to dissuade—they would never give up fighting demons.
What intrigued Ryuji most was something no other breathing style had—healing.
Yes, woven into Sun Breathing were techniques that seemed meant for healing, though they'd been twisted into attacks. One particular move, for instance, was far better suited to supporting someone than stabbing them.
"When Yoriichi first developed breathing technique, I doubt it was meant solely for killing. What a shame. Without him, the style might have evolved into something like Ripple Breathing from JoJo—a balanced art of offense, defense, and restoration."
Compared to Ripple Breathing, Sun Breathing was utterly outclassed in longevity and vitality. Ripple extended life, healed wounds, strengthened both body and spirit. In contrast, demon slayer breathing shortened lives dramatically—look at Shinobu, whose injuries even Ryuji's Dragonfire couldn't heal.
But when it came to raw destructive force, Ripple couldn't compete. If two equally talented people trained for a year—one in demon slayer breathing, the other in Ripple—the breathing user would win easily. Three years, five years… same result.
After ten years, though? Different story. The breathing user would be dead or crippled, while the Ripple user wouldn't have aged a day—likely just reaching their peak.
"A quick path versus slow mastery…" Ryuji mused, stroking his chin. "If I could develop Ripple Breathing here, maybe I could reduce my own Blood Qi Art's side effects…"
But the thought quickly soured. This world might not have the foundation to make it possible.
"Looks like I'll have to summon Yoriichi himself to study with me."
With that decision, ideas began to form—ways to adjust breathing to reduce the damage of high-speed Blood Qi circulation, to draw out energy more efficiently.
Too advanced for Tanjiro, though. The boy's head was hard, and his education limited. Best to bring in an expert.
He casually tossed a ball of Dragonfire at Tanjiro, healing his body, then waved a hand. A sphere of golden light formed in his palm, and from it stepped a tall young man with maroon hair and eyes to match, his expression tinged with melancholy.
"This… is the living world?"
The moment Yoriichi appeared, he looked around quietly. Ryuji immediately felt a gentle aura—and beneath it, a faint sense of danger.
It surprised him. His own strength far surpassed that of the Demon Slayer Corps, yet this man could make him feel threatened. Not a life-or-death danger like the monsters he'd fought before, but enough to set his instincts on edge.
"As expected of the pinnacle," Ryuji murmured. "To make even me feel danger…"
He extended his hand.
"Hello," Ryuji said with a faint smile, "I'm the one responsible for saving the world in this era. And the young man beside me—he's the one who will, in your stead, complete the mission of killing Muzan Kibutsuji in this era."
The moment Yoriichi heard the name Muzan Kibutsuji, a spark lit in his eyes. For years—lifetimes—he had carried the weight of failing to end Muzan. The thought of finishing that task himself had never left him.
"So… you brought me back from the underworld because you want me to help you kill him?"
Yoriichi's grip tightened around the hilt of his Nichirin Blade, his gaze firm. But when his eyes landed on Ryuji, something in them widened. The man radiated a quiet, suffocating danger.
"With your strength," Yoriichi said slowly, "you should be able to kill Muzan yourself… shouldn't you? Your name, after all, has reached even the afterlife."
He couldn't help asking. In the realm of the dead, he had often wondered about this man—what kind of person could scour the earth of demons and yokai alike? Now, standing before him, the answer was clear: someone who could make even him feel threatened.
Ryuji didn't comment on the observation. Instead, he stated his aim plainly.
"Muzan is a coward. The chances of catching him are close to zero."
His tone was dead serious. In Ryuji's mind, Muzan had probably already decided to vanish for centuries, never again showing himself in force. And if he hadn't decided that yet, once he learned Ryuji existed, he certainly would.
"He'll hide," Ryuji went on. "And once a man like that chooses to hide, finding him becomes almost impossible. But… if we can teach everyone the breathing arts, that changes the game. As you know, the current breathing styles take a heavy toll on the body. But if we could develop a breathing style together—one that extends life, heals illness, and still carries the scorching power of the sun—then even if Muzan emerges centuries from now, it won't matter."
He didn't say outright that he wanted to study with Yoriichi—such collaboration was inevitable. He also had his own motives.
If a breathing style could be adapted to other worlds, it would mean fewer worries about the health of certain women in his care. Even if cross-world application proved impossible, it would at least strengthen Shinobu's frail body—so she wouldn't collapse after barely a moment in his arms.
"This…"
Yoriichi's eyes brightened. He could already see the brilliance in Ryuji's plan. If everyone learned a breathing style, then everyone could fight back against Muzan. No more helpless slaughter. And if it could prolong life… there would be no shortage of people eager to learn.
Even if Muzan fled the country, his end would be inevitable.
"Agreed," Yoriichi said without hesitation.
Ryuji chuckled softly and waved a hand. In a shimmer of light, a woman appeared at Yoriichi's side.
Without a moment's pause, Yoriichi pulled her into his arms—it was his wife.
"Consider this your… employee benefit," Ryuji said. "While we work together, you can stay with her. But don't go outside."
"Ah? Why not?"
Though his heart was entirely with his wife, the thought of taking her home had already formed in his mind.
"Because Muzan is a coward. If he learns you're alive, he'll vanish without a trace."
Yoriichi paused, then exhaled a resigned sigh.
"You're right."
He had wanted to face Muzan again, but the truth was, he had never managed to find him. The demon had hidden for his entire life—only sending Yoriichi's own brother, now a demon, to meet him at the very end.
"I understand."
He took his wife's hand, his gaze shifting to Ryuji—and then to the curious young man studying his hanafuda earrings. That was when he noticed: not only did the boy wear earrings like his own, but the air around him carried the rhythm of his breathing style.
"Are you a descendant of Sumiyoshi Kamado?" he asked, the question slipping out before he could stop himself.
And in the same instant, he felt it—that this child would complete the task he had left unfinished.
"He's the one who will kill Muzan in this era," Ryuji said. "While we work, we'll train him as well. Haven't you noticed? His spirit is a lot like yours."
Yoriichi didn't answer, but his wife did.
"Yes," she said softly. "There's a warmth about him—just like you."
Ryuji glanced at her without comment, setting a small jar of fruit wine and some snacks in front of her.
"Madam, would you mind finding a few leaves to use as cups?"
~~~~~~~~~~
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