The woman whose reunion with Yoriichi had been interrupted didn't complain; instead, she cheerfully went off in search of leaves. Ryuji, meanwhile, began a deep discussion with Yoriichi.
Tanjiro, staring at the man who had just spoken his ancestor's name, was about to speak—only for Ryuji to press a firm hand on his head, forcing him back down.
This is paid time with a teacher, Ryuji thought. Save the heart-to-heart for later.
"I've been wondering—when you created this particular move and its breathing form, what exactly were you thinking?"
Ryuji assumed the stance and breathing pattern in question. There was one part he couldn't quite understand about why Yoriichi did it this way. Yoriichi glanced over and immediately answered.
"Oh, that technique actually isn't meant to be performed like that. The breathing should go like this—Sumiyoshi must have misremembered it."
The two fell into a lively exchange.
Ryuji's knowledge was broad, especially after experiencing two "genius modes" in the past—his insights often revealed angles Yoriichi had never considered. Yoriichi, for his part, was without doubt the foremost expert on breathing techniques. Once he understood Ryuji's points and learned of all the derivative breathing styles in this era, he immediately began refining the original Sun Breathing.
Tanjiro, however, ended up as their test subject. Whenever the two had a new idea or wanted to try something, they had Tanjiro give it a go.
It couldn't be helped—both Yoriichi and Ryuji possessed almost absurd vitality. What worked for them was useless as a baseline for ordinary people. Tanjiro, being closer to normal, was the ideal test case.
And the results were remarkable. In just a few hours, they had reshaped Sun Breathing into a version perfectly suited for Tanjiro's body, even adding techniques for healing injuries.
Ryuji also gained a great deal in the process.
"So that's how it works…"
By using Yoriichi's method of manipulating blood-qi through breathing, Ryuji could now sense exactly how Yoriichi stabilized bleeding, and—thanks to some of his own theories—he'd learned ways to channel blood-qi more safely through the heart.
"This really is impressive, Yoriichi—you're a genius."
He couldn't help praising him. While he wouldn't compare him to Roger, whose teachings had drastically boosted his own combat power, Yoriichi's mastery over fine, delicate control was second to none.
If not for the fact that Yoriichi couldn't use Ryuji's style of blood-qi, Ryuji would have begged him to help improve his own Blood-Qi Secret Arts.
Yoriichi, however, responded sincerely: "No—you are the genius here…"
To him, Ryuji's breadth of martial knowledge was like facing an entire library. Not only did he possess multiple powers, each was honed to mastery. If it weren't for Ryuji's youthful aura, Yoriichi would have assumed he was an eighty-year-old scholar who had spent his entire life studying.
And yet Ryuji looked barely past twenty—this went beyond genius. He must, Yoriichi thought, be a scion of a great clan, with the resources to learn so much.
"Hahaha… nothing like that. I just had some unusual encounters," Ryuji replied modestly. His knowledge had largely come from the world-granted "genius mode" he'd earned by slaying monsters. Truly internalizing it all was another matter—most of it only came to mind when prompted.
Still, after two such transformations, he had "mastered" it in practice. If not for the current crisis, he would gladly have spent a year or two training before facing the final monster—those genius modes were that valuable.
"Alright, enough mutual flattery," Yoriichi's wife interjected with a smile. "What about that thing you two mentioned earlier—is it done?"
Her concern was that her husband could rest without regrets.
"We've completed a breathing method with no harmful side effects—one that can even extend a person's lifespan," Yoriichi explained. "But it's difficult to learn, and the results vary for each individual."
With Ryuji's insight into human blood-qi and some modern medical knowledge, the concept had quickly taken shape. This style didn't require exceptional talent—just the ability to control one's breathing—but it was hard to persist with. Benefits would only appear after a year or two of practice.
Ryuji suggested splitting it into multiple, simpler breathing styles:
One to boost energy and stamina during work, with a built-in safeguard that made it impossible to overstrain the body.
Another to induce deep, restorative sleep.
Others to repair the body and counteract strain from the first style.
Even specialized versions to heal the kidneys or maintain stomach health.
The full form was complex, but broken down, it could be taught widely.
"Everyone's needs are different, so mass adoption will be tricky," Ryuji said. "But if we can embed breathing techniques into the fabric of this world, the rest will take care of itself. The Demon Slayer Corps could shift from killing demons to studying and refining breathing methods."
He had no interest in letting one family bear the sole burden of demon hunting. If it were him, he'd hand the techniques to the shogunate and flood the land with them, forcing Muzan into a corner.
Unfortunately, the Ubuyashiki family's only goal was to kill Muzan—they'd never agree to a plan that might spook him into hiding.
As for Ryuji's personal gains from Yoriichi? A perfected Sun Breathing tailored to his own style, and a set of breathing secrets to stabilize or accelerate his Blood-Qi Arts. More than enough for his needs—his focus and stamina in battle were finite, after all.
"That's great… By the way, Yoriichi, you'll have about a week to roam freely. Feel free to explore Corps headquarters. If we're lucky enough to run into Muzan, I'll summon you."
Ryuji's lips curled into a smile. Tanjiro had already undergone a major evolution—his custom Sun Breathing was at eighty percent of the original's power with almost no side effects. Ryuji was even considering making him dress like Yoriichi and dye his hair red, just to see if Muzan could resist taking the bait.
"Understood," Yoriichi nodded.
At that moment, the sky began to brighten—
—And then both men turned toward Nezuko's box.
From inside came the sound of steady, powerful breathing… and the unmistakable sensation of a vast life-force awakening. The rhythm was identical to their perfected Sun Breathing—yet subtly, strangely different.
Under that breathing, the morning sunlight seemed to sharpen, pouring into the box in endless streams.
"No way…"
Ryuji's eyes widened in realization.
A heartbeat later, the wooden box burst apart.
A tall figure stepped out, back to the sun, a peculiar smile on her lips. Her hair swayed and curled like living tendrils in the wind. The blinding light behind her made her face impossible to see—but they knew it was Nezuko. And they could see—
Nezuko tugged at the front of her loincloth, lifting it slightly—though nothing was exposed, as another layer of fabric lay beneath.
~~~~~~~~~~
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