Why the Kamado family became the inheritors of Sun Breathing was something that happened after Yoriichi encountered Muzan and allowed him to escape.
At that time, Yoriichi was blamed and banished from the Demon Slayer Corps for letting Muzan escape, allowing Tamayo to go free, and for the murder of the then-Master of the Corps by his brother.
Losing both refuge in the Demon Slayer Corps and the identity of a Demon Slayer, Yoriichi, in a state of loneliness, encountered two ancestors of the Kamado family—a husband and wife pair who were being attacked by demons.
As Yoriichi saved them, he ultimately became friends with them.
Two years later, Yoriichi visited them again and told his story to the husband, who had become his closest friend.
At the husband's request, he later demonstrated Sun Breathing.
As Yoriichi demonstrated every sword form of Sun Breathing, they were deeply imprinted in the mind of the husband, without any loss or distortion.
As a result, the Kamado family's Hinokami Kagura was born.
Before parting, Yoriichi also gifted his hanafuda earrings to the husband.
However, the Sun Breathing personally demonstrated by Yoriichi was passed down in the Kamado family along with the pair of hanafuda earrings.
Because of this, although the Kamado family had been an ordinary charcoal-selling family for generations, they were also the sole inheritors of Sun Breathing in the world.
Otherwise, why would Tanjirou's father understand the Transparent World?
It was because he was proficient in Sun Breathing since childhood, which inadvertently led to his mastery and awakening of the Transparent World.
Unfortunately, Tanjirou's father had been very ill and bedridden all year round before passing away three years ago due to his illness.
Given this, considering his talent for awakening the Transparent World without a teacher, if he had joined the Demon Slayer Corps and formally practiced swordsmanship, he would have definitely become a stronger swordsman than Gyoumei, the strongest in the Corps.
"Oh, I see…"
After listening to Riezel's explanation, Tanjirou remained stunned for a long time, still unable to snap out of it.
Truthfully, Tanjirou's reaction was understandable.
For a plain charcoal-selling family to actually be the inheritors of the strongest breathing style and the ancestors of all breathing styles, the feeling was simply comparable to the shock of, 'Oh, I'm actually born rich.'
*Ding—ling—*
At the perfect moment, a breeze blew, causing the hanafuda earrings hanging from Tanjirou's ears to sway gently, emitting a tinkling sound like wind chimes.
The pleasant sound not only made Riezel feel nostalgic but also awakened Tanjirou from his daze.
"...I see now... this must be fate, right?"
Tanjirou muttered and clenched his fists, determination evident in his eyes.
As the creator of Sun Breathing, Yoriichi failed to kill Muzan, but because of this, he encountered Tanjirou's ancestors by chance and passed down Sun Breathing.
Moreover, as inheritors of Sun Breathing, the Kamado family was coincidentally attacked and almost wiped out by Muzan, leaving only Tanjirou and his younger sister, who was turned into a demon.
Since Tanjirou wanted to turn his younger sister back into a human, he had no choice but to search for Muzan to obtain a method to revert demons back to humans.
With all these coincidences combined, Tanjirou truly feels the weight of fate.
He must defeat Muzan.
Regardless of whether it was to fulfill Yoriichi's regret as the inheritor of Sun Breathing or to seek revenge as the eldest son of the Kamado family and find a way to restore his younger sister back, Tanjirou felt he must defeat that Demon King.
As Riezel looked at Tanjirou, whose eyes were filled with determination, he suddenly spoke.
"If you wanna turn your little sister back into a human, maybe I can help."
As if hit by lightning, Riezel's sudden statement left Tanjirou completely dumbfounded.
"Wh-What...?"
Although Tanjirou couldn't believe his ears, Riezel had indeed said it clearly.
"I can help you turn your younger sister back into a human."
Riezel's casually repeated words plunged Tanjirou into unprecedented excitement.
"Re-Really? Is what you said true, Mr. Tsugikuni?!"
Tanjirou couldn't help but grab onto Riezel's clothes in desperation.
"Really."
Riezel wasn't just saying things randomly.
"Humans become demons because Kibutsuji Muzan injected his blood into them, transforming their body constitution." Riezel explained. "So theoretically, if we can obliterate his blood inside their body, it should be possible to turn them back into humans."
Demons not only fed on humans but also increased their strength by consuming Muzan's blood—the more blood he gave them, the stronger they became.
However, if the power within the blood were too strong, weaker demons might not be able to handle it and could end up self-destructing. Conversely, those who were able to handle it could become even stronger than Upper One Kokushibou by relying solely on Muzan's blood.
From this, it could be seen that Muzan's blood was the source of the demons' power. In other words, by decomposing his blood within their own, they should revert to being human.
For Riezel, who possessed decomposition magic, this was entirely achievable.
"Mr. Tsugikuni... I... I..."
Tanjirou stuttered, overcome with emotion, tears streaming down his face.
"Alright, don't look at me like that, I just don't want to see your little sister get hurt because of me."
Riezel shook his head and patted Tanjirou's shoulder.
"Later, I plan to kill Kibutsuji Muzan myself. Once he's gone, all the demons in the world will disappear with him. If your little sister dies because of this, I'll feel guilty."
In fact, the original work didn't really mention what happened to the demons after Muzan died.
Riezel wasn't sure if they would die along with him or if they would revert to being human. If it was the latter, it would be a good thing, but if it was the former, Tanjirou would have to face the loss of his closest family.
With this in mind, Riezel decided to do a good deed and save Tanjirou's younger sister first before confronting Muzan.
"Thank you! Mr. Tsugikuni! Thank you very much!"
Tanjirou repeatedly bowed to Riezel, tears streaming down his face, crying in a rather unsightly manner.
"Alright, let's go." Riezel said with a smile. "Take me to your little sister."
"Yes!" Tanjirou replied loudly.
===
In the Swordsmith Village, Tanjirou lived in the collective dormitories provided for members of the Demon Slayer Corps.
The dormitories varied in size—some were large, some small, some accommodated only two to three people, while others could house around ten. Additionally, they were located on the outskirts of the village, clearly newly constructed.
Meanwhile, Tanjirou resided in one of the smaller dormitories, which could accommodate only three or four people. Naturally, he shared it with Zenitsu, Inosuke, and his younger sister, Nezuko.
Under Tanjirou's guidance, Riezel arrived in his dormitory and met his younger sister, Nezuko, who had always been kept inside a wooden box.
Nezuko was a girl who seemed to be around the same age as Mediste, about fourteen years old. With waist-length jet-black hair, she wore a pink kimono with a black haori over it and had a bamboo tube in her mouth.
Riezel must admit, her appearance was cute, clearly a rare beauty in the making.
At this moment, Nezuko was sleeping soundly on the futon laid out on the tatami mat.
"I'm back, Nezuko."
Tanjirou knelt beside the futon, reaching out to gently stroke his younger sister's head with a gentle and serene expression that Riezel had never seen before.
"Is she asleep?"
Riezel asked as he observed Nezuko for a moment.
"Yes..." Tanjirou unconsciously lowered his voice. "Since becoming a demon, Nezuko has often slept like this, spending most of her time sleeping, seemingly maintaining her strength through sleep and suppressing her instinct as a demon, refraining from eating humans."
It must be said that Tanjirou's younger sister was quite impressive.
After turning into a demon, most people were usually controlled by uncontrollable violent tendencies, attacking humans on sight and gradually losing their humanity, even their memories of being human.
Nezuko was the only demon who, through sheer willpower, overcame the instinct to eat humans and retained some of her humanity thanks to Tanjirou's calls.
Even though she was the only demon in this world who had never eaten a human, her presence had caused conflicts within the Demon Slayer Corps.
After all, Tanjirou being together with a demon, and even supporting the demon, was a major taboo within the Corps.
Even someone as strong as Yoriichi had been condemned by the Demon Slayer Corps for sparing Tamayo, let alone Tanjirou, a mere ordinary member. Because of this, the Corps had once considered executing his younger sister, and Tanjirou himself had also almost been executed.
If it weren't for Giyuu vouching for him and Shinobu later showing favoritism toward Tanjirou, along with Kagaya having some understanding of Nezuko's situation and allowing her existence, perhaps these siblings would no longer be part of the Demon Slayer Corps.
Even so, most members of the Demon Slayer Corps still refused to acknowledge Nezuko. Some even refused to accept Tanjirou, despite Nezuko proving at the last Hashira meeting that she didn't eat humans.
Luckily, there were now quite a few Hashira who acknowledged Tanjirou.
Kyoujurou becoming Tanjirou's mentor naturally implied recognition of Nezuko.
Mitsuri never harbored biases against Tanjirou and always held him and his younger sister in high regard because she was kind-hearted.
Muichirou seemed to consider Tanjirou and Nezuko's existence insignificant and chose to ignore them.
Gyoumei seemed to be in a state of observation, with his thoughts unknown.
As for Tengen, while he couldn't exactly be said to acknowledge Tanjirou and Nezuko, he hadn't insisted on rooting them out either.
Only Sanemi and Obanai, the two with the worst tempers, worst personalities, and lowest popularity within the Demon Slayer Corps, still held a grudge against Tanjirou until now.
Of course, as someone who knew the original work, Riezel naturally held an approving attitude toward Nezuko.
He had come here to help her, which in itself was proof.
"Alright, I'm going to begin."
Riezel knelt down on one knee and placed his hand on Nezuko's forehead.
"Please... I beg of you!"
Tanjirou bowed deeply to Riezel, his face filled with nervousness.
Without bothering with Tanjirou further, Riezel began to use decomposition magic.
*Bzzzt—*
Crimson waves soon appeared.