The Raikage lineage, they used the Lightning Release Chakra Mode to continuously temper their bodies, making their physical forms, even without the use of ninjutsu, arguably the strongest in the ninja world. The Third Raikage, when later revived by Edo Tensei, could withstand Naruto's Wind Style: Rasenshuriken without using ninjutsu—his physical body was practically invincible. Although an Edo Tensei body could regenerate, he had, in fact, blocked the Rasenshuriken with his raw physique without relying on ninjutsu.
As Uchiha Nagi walked, he pondered. Based on the fragmented memories from his past life, Breathing Styles weren't without their weaknesses. It seemed they absolutely had to be coordinated with one's breath during use. Later on, during the fight against Upper Rank Two, the demon's Blood Demon Art could introduce substances into the air, perfectly countering Breathing Styles.
However, it wasn't a major issue. Just like senjutsu, every technique had its flaws. But any weakness could be overcome through training. The First Hokage had a perfect Sage Mode, allowing him to absorb natural energy while fighting. And Madara Uchiha was even more incredible. Upon his revival, without Sharingan or Rinnegan, he utterly dominated everyone. Then, when sealed by Shukaku within a pyramid, he simply activated Susanoo to break free. Activating Susanoo without a Mangekyo Sharingan? Was that something a human could even do?
Uchiha Nagi wondered: could Breathing Styles be manifested in a way similar to Sage Mode? Sage Mode, after all, was the strongest combat mode, created by balancing natural energy, one's own energy, and spiritual energy. Breathing Styles, on the other hand, seemed to be an application combining natural energy and one's own energy, and not even a complete version at that.
Therefore, the Hashira of this world possessed physical bodies that surpassed human limits, enabling them to battle immortal demons with their flesh and blood, and to use various Breathing Styles combined with sword techniques to contend with Blood Demon Arts.
"Hmph~ It seems I can give it a try," Uchiha Nagi resolved. Once his body was back to one hundred percent, he would begin his attempts. He would try to combine his physical energy, spiritual energy, and Breathing Styles—a trinity—to create his own unique Sage Mode. When Breathing Styles reached their peak, a Mark would appear. Senjutsu users developed facial markings. Wasn't the similarity too coincidental?
He had no lineage to draw upon, no access to the three great sage regions, and thus no way to learn how to cultivate natural energy. Besides, he didn't want to learn it that way. Hadn't he seen how high the mortality rate was? For a human body to forcibly learn the sage arts of summoning animals—it would be strange if the death rate wasn't high. Only an idiot would gamble on those kinds of odds.
But the Breathing Styles of the Demon Slayer world were created by humans, and thus were the sage arts most suited for humans themselves.
Thinking of the strongest Breathing Style, Sun Breathing, Uchiha Nagi's gaze on Tanjiro, who was walking beside him, grew intense, leaving the younger boy utterly bewildered.
"South-southeast! South-southeast!"
An annoying crow circled relentlessly above Tanjiro's head before finally landing on his shoulder, still cawing incessantly.
"I know, I know! Stop shouting already! Ugh, my ears are going to die from your noise," Tanjiro grumbled, rubbing his ears. He turned to Uchiha Nagi and explained, "This is a Kasugai Crow. They're companions for Demon Slayers. Everyone is paired with one for communication."
"Mm, understood," Uchiha Nagi nodded. Because this world doesn't have cell phones...
Suddenly, a loud cry pierced the air— "Please~" "Please, marry me! I'm going to die soon~" A young man with yellow hair, wearing a yellow haori, was kneeling before a girl, begging her to marry him. "Please, I beg you! I'm going to die soon! Please marry me before I die~" Tears and snot streamed down the yellow-haired boy's face as he sobbed uncontrollably, as if he were about to be sent to a battlefield the very next moment.
[He's here. The Thunder Breathing user, Agatsuma Zenitsu...]
Uchiha Nagi quietly observed the boy. He was the same age as Tanjiro, a Demon Slayer who used Thunder Breathing. Currently at the lowest rank, yet he possessed the potential of a Hashira. The Demon Slayer Corps was divided into ten ranks: Kinoe, Kinoto, Hinoe, Hinoto, Tsuchinoe, Tsuchinoto, Kanoe, Kanoto, Mizunoe, and Mizunoto. New recruits all started at the lowest rank. Above Kinoe were the Hashira, humanity's current highest combat power.
Uchiha Nagi didn't intervene, watching as the two innocent youths had their comically awkward first encounter. In the future, they would become comrades who would face life and death together.
After helping the unfamiliar girl escape Zenitsu's desperate pleas, Zenitsu latched onto Tanjiro, looking utterly terrified of dying. It was impossible to imagine that once asleep, this very person would transform into a powerful master of Thunder Breathing.
"Eeeek!! I'm not going! I'm not going~ No matter what you say, I'm not going!" Zenitsu wailed, clinging desperately to Tanjiro's leg.
"Zenitsu, you're not going? Why are you so shameless? Think about why you became a Demon Slayer in the first place!" Tanjiro said, exasperated.
"I never wanted to become a Demon Slayer in the first place! I was tricked by a woman and ended up in debt! The old man who paid it off for me was a Cultivator, and then it was just hellish training, training, and more training every single day!" Zenitsu clutched his head. "I'm definitely going to be eaten by a demon! My brains will be sucked out through my ears before I even die! I don't want to! Save meeee~~~" he shrieked while crying.
Beside him, his own Kasugai Crow—a distinctive sparrow—perched on Tanjiro's shoulder, chirping complainingly. Watching Tanjiro nod and respond, Uchiha Nagi could roughly guess the content of the sparrow's complaints: its master was a coward, terrified of demons, and wouldn't go when one appeared...
Uchiha Nagi was quite envious of Tanjiro's ability to communicate with animals. His own crow could speak, but this sparrow certainly couldn't. Apparently, Zenitsu, like Tanjiro, possessed some peculiar ability to communicate with animals. Oh, right, this guy also had incredibly sharp hearing.
"To the southeast, in a house, the target has appeared. Members of the Demon Slayer Corps have already rushed over. Let's hurry!" Tanjiro announced.
And so, Tanjiro dragged the still-wailing Zenitsu along, with Uchiha Nagi accompanying them. After some time to cool down, Zenitsu still followed them with an expression that screamed he'd rather be dead.
After walking for several hours, the three of them arrived outside a house. Tanjiro spotted two children in the woods and ran over to ask them what was wrong. Just then, a body was flung out of the house.
Uchiha Nagi rushed over. The man who had been thrown out was on the verge of death. He barely managed to utter a few words before he stopped breathing. With such injuries, not even medical ninjutsu could have saved him.
"If only we had arrived sooner. Damn it, we couldn't save him," Tanjiro said, bringing the two children over. He explained to Uchiha Nagi that they were the ones involved; their older brother had already been dragged into the house by a demon.
Drawing his Nichirin Blade, Tanjiro was about to charge in but was stopped by Uchiha Nagi.
"A wooden house? We could try burning it down. They're afraid of sunlight," Uchiha Nagi said, his hands beginning to form seals. After two days, his White Zetsu arm had healed perfectly. He could move it now, though the bone connection wasn't quite at full strength yet.
"Fire Style: Great Fireball Jutsu!" Uchiha Nagi unleashed his signature fire ninjutsu, intending to burn the house and force the demon inside out. Having spent so long in the ninja world, he subconsciously felt that entering an enemy-occupied house was walking into a trap.