As night fell, a doctor came to examine the group and prescribe medication. However, Uchiha Nagi gestured that it wasn't necessary, as he himself was a doctor. He activated the Mystical Palm Technique, and a vibrant green chakra instantly sealed a wound.
Tanjiro and the others stared, dumbfounded.
Tanjiro, never one to keep his thoughts to himself, asked directly, "Nagi-nii, if you can heal, why didn't you treat us on the mountain earlier?"
Uchiha Nagi shook his head. "The Mystical Palm Technique uses chakra to accelerate human cell growth. However, the number of times human cells can divide in a lifetime is limited. Rapidly forcing them to divide isn't actually ideal. If there's no life-threatening danger, it's best to let the body's injuries heal naturally. That's the optimal way." He added, "Moreover, you're all still in your adolescent growth phase. Overusing the Mystical Palm Technique would consume your cells' regenerative power, which is tantamount to depleting your potential."
Tanjiro, who could sense a person's emotions and desires, knew Uchiha Nagi wasn't lying and nodded in understanding. "I see, I understand now. By the way, Nagi-nii, about that chakra you mentioned, I've been trying to refine it for days but can't seem to get the hang of it. Could you explain it to me again?"
"Mm, alright—" Uchiha Nagi nodded.
"What's chakra?" Zenitsu asked from the side, with Boar-boy Inosuke also looking on with a curious expression.
"Heh~" Uchiha Nagi chuckled lightly. Coming to this world, which was close to modern times, and getting along with these younger fellows was quite pleasant. A part of his own heart, frozen for so long, was slowly beginning to thaw. So, he didn't mind teaching them a few things. "Chakra is this. Watch closely. Shadow Clone Jutsu—"
Poof! After a puff of smoke, several Uchiha Nagis appeared together in the living room, each a solid Shadow Clone.
"Sugoi—" Zenitsu was the first to rush over, touching Uchiha Nagi's Shadow Clones, examining each one carefully, and finally confirming it wasn't an illusion.
"Ooh ooh—Several mes? Haha, I win, I win....." Boar-boy Inosuke, also looking curious, touched the Shadow Clones. Then, for reasons unknown, he suddenly put his hands on his hips and burst into wild laughter.
"Want to learn? I'll teach you," Uchiha Nagi said with a smile.
"Mhm, mhm, we want to learn~" chorused all three. The three of them sat before Uchiha Nagi like obedient children. Nagi glanced at Boar-boy Inosuke; this fellow didn't seem too difficult to get along with either. He had probably never interacted much with others, having grown up alone in the deep mountains. So, a little kindness was all it took to become his friend.
"Ahem, then I'll begin. Chakra is an energy produced by the perfect fusion of various component energies controlled by the human body. It's the energy source for performing ninjutsu, genjutsu, taijutsu, or for forming it into threads to bind opponents or sever substances also composed of chakra. This energy is composed of: one, physical energy drawn from each of the body's approximately 130 trillion cells; and two, spiritual energy, which is cultivated through extensive training and accumulated experience."
"......."??? "What are cells?" "What's spiritual energy??" Inosuke even started counting on his fingers. "How much is 130 trillion? I only know how to count from one to ten."
Uchiha Nagi: "......." It seemed he had a long road ahead of him. Faced with these three, who were less knowledgeable than even Ninja Academy kindergarteners, Uchiha Nagi felt a headache coming on at the thought of teaching them to extract chakra.
However, the three were clearly very interested in a ninjutsu like the Shadow Clone Jutsu, which created solid clones. Even if they didn't understand, they still sat attentively and listened. The scene was only interrupted when Tanjiro's sister, Nezuko, emerged from the wooden box.
Even Uchiha Nagi was momentarily struck by Nezuko's beauty—a perfect blend of purity and loveliness, especially her dazed, innocent eyes, which seemed as pure as if she hadn't been touched by the mortal world, inspiring an instinctive desire to protect her. However, it was just a fleeting moment of admiration. Uchiha Nagi wasn't someone who hadn't seen the world; as a transmigrator, he had seen countless beauties on the internet and quite a few in the Naruto world as well.
In contrast, Zenitsu, timid as a mouse, transformed into some kind of tengu, wagging his tail as he chased after Nezuko, scaring her in the process. Boar-boy Inosuke nearby was also clearly a bit awestruck by Nezuko, though he didn't seem to know how to express it.
The group spent a harmonious, albeit noisy, evening together. Uchiha Nagi also felt incredibly at ease, even unintentionally falling into a deep sleep.
The next morning, when Tanjiro and the others opened their eyes, Uchiha Nagi was already in the courtyard, practicing the Breathing Styles he had just learned. "Beast Breathing, First Fang: Pierce!" "Water Breathing, First Form: Water Surface Slash!" Two Shadow Clones, wielding swords, sparred with each other. Inosuke, who had just come out, watched in utter shock. "You..... Why do you know my Beast Breathing??"
Smack! Zenitsu slapped Inosuke on the head. "Idiot! Call him Nagi-nii! Oh, right, Tanjiro?" Tanjiro nodded. "Nagi-nii can learn a Breathing Style just by seeing it once. He must have seen you use it in the demon house earlier, so he learned it."
"So... so amazing." Boar-boy Inosuke was completely petrified, frozen in place.
"How are your injuries? If they're better, you can join my Shadow Clones for morning practice. Don't worry about hurting me; if a clone is injured, it will just disappear. The main body will be fine," Uchiha Nagi said.
"Ooh ooh, no problem! Then here I come, Nagi-nii! Let's settle this! Pig Assault—" Boar-boy Inosuke roared, grabbing his two serrated Nichirin Blades and charging forward. Uchiha Nagi, intending to learn more of this world's Breathing Styles, had one Shadow Clone practice with him. Tanjiro and Zenitsu also joined the morning training. They didn't go all out, as it was just daily practice, so the intensity wasn't too high, and they weren't worried about straining their bodies.
Uchiha Nagi intentionally went easy on the three to train them. Their combat styles were too one-dimensional. Letting them experience the Naruto world's way of fighting would more easily improve their strength. The three youngsters, in terms of overall strength, might only be at the level of genin in the Naruto world. They didn't know ninjutsu, the three basic academy techniques, or genjutsu, let alone strategies involving poisons or complex tactics. However, in terms of raw combat power alone, they might possess taijutsu skills comparable to Neji during his fight against the spider demon, Kidomaru, or even surpass it. Who could have guessed that Breathing Styles were such a jack-of-all-trades existence? In the long-term fight against demons, the Demon Slayer Corps continuously used Breathing Styles to counter unknown Blood Demon Arts, constantly improving them. With all sorts of bizarre Blood Demon Arts out there, Breathing Styles had gradually evolved to become adaptable enough to deal with them. It could be said that, without considering genjutsu and secret techniques, Breathing Styles could versatilely contend with the ninjutsu and taijutsu of the ninja world.