"You've already helped me immensely by sewing the blood vessels and muscles of my arm back together." Uchiha Nagi was soaked to the skin, looking as though he'd just climbed out of a river. Even with multiple Shadow Clones, each could only operate with one hand, which wasn't nearly as efficient as having two hands working in concert. Besides, Tamayo's surgical skill was remarkably adept.
Uchiha Nagi flexed his fingers. Though the surgery had just concluded, he could already control them. The heaven-defying properties of the White Zetsu cells were truly something to marvel at. He could even feel an intense itching at the site of the bone, in the muscles, and along the blood vessels and nerves—a clear indication that the cells were regenerating at an astonishing rate. His blood was already circulating vigorously through the White Zetsu arm, flowing back and forth.
The White Zetsu arm absorbed nutrients and chakra from his bloodstream, its cells proliferating wildly to mend the severed limb. Nagi was confident that the bone at the point of reattachment would heal in less than a day, and within three or four days, he'd have perfect, normal use of his arm again.
More importantly, there seemed to be an inexplicable substance within the White Zetsu cells, slowly circulating through his body with his blood. His Mangekyo Sharingan, which had been heavily strained from using its dojutsu, felt noticeably less fatigued. A cooling sensation began to flood his eyes; even his severely taxed left eye was slowly starting to recover. He believed he wouldn't have to wait an entire year to use it again.
"I've received a reply," Tamayo said, her voice calm as ever. "Mr. Ubuyashiki Kagaya is very pleased to welcome you and extends an invitation for you to visit the Demon Slayer Corps headquarters. Please rest for a few days. Once your injuries have healed, you can set off with Tanjiro."
"Mm, thank you very much," Nagi replied.
It's a matter of quid pro quo, Nagi thought. He wanted to learn this world's Breathing Styles, so joining the Demon Slayer Corps and lending his aid was perfectly justifiable. Besides, compared to the ninja world, where one could be ambushed by a squad of shinobi with diverse abilities at any given moment, demons in this world were far fewer in number. Most of the time, it was the Demon Slayer Corps who held the numerical advantage, turning the tables on the demons. And as long as he was under the sun, he could completely relax, with no need to worry about sneak attacks.
In the ninja world, whether you were eating a meal, sleeping, or even just playing a game of cards with a woman, you constantly had to be wary of poisoning or ambushes. Especially in Konoha, Nagi could never truly feel at ease, not with a certain notorious figure always scheming to get his hands on Sharingan.
As per their agreement, Uchiha Nagi then began to exchange medical knowledge with Tamayo, explaining the principles of the Mystical Palm Technique.
"Chakra?" Tamayo mused. "Is that the fundamental energy required for your wondrous techniques? How fascinating. This is the first time I've heard of humans possessing something called chakra, aside from Breathing Styles."
She continued thoughtfully, "According to your explanation, it's extracted from human cells, which bears some resemblance to our Blood Demon Arts. However, ever since our bodies were transformed from human to demon, we likely lost the ability to extract this energy you speak of. We can only utilize Blood Demon Arts."
Uchiha Nagi elaborated, "Blood Demon Arts seem to be techniques derived from your individual genetic makeup and inherent nature, correct? Each demon cannot replicate another's Blood Demon Art. Chakra, however, can be used to perform techniques common to all humans who can wield it. But the specifics of how it might apply here still need to be gradually verified."
Since no one in this world currently possessed chakra, it was uncertain if they could even learn to use it. It was a matter requiring further research.
As for Breathing Styles, however, Uchiha Nagi was certain he could use them. Copying Tanjiro's Water Breathing had posed no problem at all. His own elemental affinities were strongest in Fire, moderate in Water, Lightning, and Wind, and relatively weak in Earth. Thus, replicating Tanjiro's Water Breathing wasn't difficult.
Furthermore, chakra affinities could be strengthened through dedicated development. So, couldn't different elemental Breathing Styles also be developed in a similar fashion? Kakashi had constantly developed his various affinities, eventually earning the moniker "Copy Ninja." Even if one had a congenitally weak affinity for a certain element, it could be significantly enhanced through diligent training.
But he was digressing. After conversing with Tamayo for a while longer, Uchiha Nagi made his way above ground. Tamayo and Yushiro couldn't venture out during the daylight hours, but Tanjiro could.
Tanjiro was in the dilapidated courtyard, tirelessly practicing his Breathing Style, coordinating it with his sword techniques to execute a series of dazzling moves.
After observing for a moment, Uchiha Nagi called out, "Tanjiro, there's something I'd like to discuss with you."
Tanjiro turned, sheathed his Nichirin Blade, and walked over. "Nagi-san, is your surgery finished?" He then looked at Nagi's arm, his eyes wide with amazement. "It's incredible! You actually possess such an ability? You can even reattach a severed hand?"
Nagi offered a faint smile. "Heh, if you want to learn, I can teach you. By the way, could I properly observe your Water Breathing technique? As an equivalent exchange, I can teach you something else."
With that said, Uchiha Nagi formed a hand seal with a single hand, and a Shadow Clone flickered into existence beside him. Then, the original Nagi and his clone performed a complex series of hand seals in unison. A moment later, a Water Style jutsu materialized before Tanjiro's astonished eyes.
"Water Style: Water Dragon Jutsu!"
A colossal dragon, composed entirely of swirling water, soared into the sky before crashing down through the nearby trees, felling a large swath of them with its immense power. The sheer scale of the technique was, in a single word—awesome.
"Hiss—Sugoi~!" Tanjiro gasped, his breath catching in his throat. "That's amazing, Nagi-san! Your... Breathing Style is incredible! And you can make clones! Are you really willing to teach me? Oh, no, I mean, you can learn my Water Breathing anytime! It's okay if you don't teach me your style~"
Uchiha Nagi chuckled. "I'm only a few years older than you. Just call me Nagi-nii. Constantly calling me 'Nagi-san' makes me feel old."
He continued, "What I use is called ninjutsu, not a Breathing Style. It requires refining a type of energy from the human body called chakra to use. This clone is called the Shadow Clone Jutsu, and it's a solid, physical clone. However, I can't guarantee you'll be able to learn it unless you can successfully refine chakra."
Tanjiro stared at Uchiha Nagi's Shadow Clone, his eyes practically burning with fervent excitement. Such an amazing ninjutsu—who wouldn't want to learn it? If he could create a dozen clones every time he fought a demon, each wielding a Nichirin Blade, then...
"Sluurp~" Lost in thought, Tanjiro nearly started drooling at the prospect.
And that ninjutsu that could manipulate water—it looked so incredible too! Even though his own Water Breathing had "water" in its name, in actual combat, there wasn't any tangible water swirling around his body; it was simply a breathing method combined with sword techniques.
Nagi explained, "Chakra is an energy created by perfectly blending the physical and spiritual energies that every human body possesses and controls. It's the power source for ninjutsu, genjutsu, and taijutsu."
"This physical energy is drawn from every single one of the body's roughly 130 trillion cells. Spiritual energy, on the other hand, is honed through extensive training and accumulated experience."
Uchiha Nagi mentally added the final, crucial piece of information: And if one cultivates senjutsu, a portion of that energy is natural energy.
Senjutsu had always been something he yearned to master. Unfortunately, without a proper teacher to guide him through the fundamentals, it was incredibly difficult to grasp through self-study alone. Even geniuses like Uchiha Madara and the Second Hokage, Tobirama Senju, had been unable to master senjutsu. They were both close relatives of the First Hokage, Hashirama Senju, who certainly wouldn't have withheld such knowledge from them. Their inability to learn it only underscored how incredibly high the threshold for mastering senjutsu truly was.
As for Naruto and Jiraiya, who came later... their senjutsu was somewhat incomplete, a half-baked version compared to true mastery.