Taiyo kamado (the older brother of Tanjiro)

Pre chapter 1

 It was the golden age. Medicine, architecture, and people were thriving little did anyone know that a true monster was about to be born. The Heian period was when this monster would be born, his name would be none other than Muzan Kibutsuji. This monster would be born with a disease that should have killed him at birth, but luck was on his side, and he survived. The child was thought to be dead before the age of 25. Muzan had a stroke of luck when a doctor came with a possible cure. Once Muzan had taken the cure he did not feel any difference, he then murdered the doctor only to realize that he was getting stronger. Though he had gotten stronger, Muzan could not walk in sunlight and had an unpleasable diet of human flesh. Since Muzan had killed the doctor, he could never get his hand on the ingredient he needed to achieve his perfect state, the blue spider lily. Over the centuries Muzan grew stronger, he overcame every type of decapitation and was even closer to reaching his perfect state.

Muzan started to create demons, in hopes one demon could conquer the sun and he could absorb them to finally gain the perfect state. Centuries had passed by and muzan had no luck.

Some time had passed and it was the Sengoku era, Muzan had discovered about breath uses who were extremely strong demon slayers who were easily killing his demons and one day could even destroy him.

One day Muzan stumbled upon a demon slayer called Michikatsu tsugikuni, who was afraid of dying. Muzan then offered him to become a demon since Muzan also wanted one of these extremely powerful demon slayers under his command.

Some days after Muzan met the creator of the breathing techniques. Muzan had not known this and out of confidence attacked this man. Little did Muzan know that this man could end him in one attack.

That man was Yoriichi tsugikuni, a child prodigy whose brother was turned into a demon by Muzan.

In one strike man fell to the ground holding his decapitated head with what was left of his arms.

Muzan was not regenerating and for the first time in more than 400 years, Muzan felt fear, fear that he had felt when he was a human. The man came up to Muzan and asked him "What is the value of a life to you?" Muzan didn't respond to the man and was plum red the man noticed this and unsheathed his blade to finish Muzan, but Muzan split into multiple pieces and escaped.

Yoriichi had been banished from the demon slayers since he took responsibility for his brother's demonization and his failure on defeating the progenitor of demons. Yoriichi continued to regret and would eventually hand his earrings and breathing technique (Sun breathing or hino koqu) to his friend Sumiyoshi Kamado. Sumiyoshi vowed to preserve the earrings and the breathing technique by passing it on to his descendants. Sumiyoshi new it would be the last time he would see his friend and shouted telling him to not look down on himself, Yoriichi froze and looked back to his friend saying one word, thank you (ari gaato).

Yoriichi's brother had been turned into the demon named Kokushibu. The brothers reunited after around 60 years and Kokushibu looked at his younger brother in disgust, seeing his wrinkled face, his black hair turned white, and his eyes were pure white (he was blind). Kokushibu looked at his brother with all six eyes noticing him cry, Yoriichi said "my sympathies, my brother". Kokushibu was in disbelief at the fact that his brother was still alive. As Yoriichi finished crying, Kokushibu unsheathed his sword and got in a stance ready to kill his brother, suddenly Yoriichi got in a stance ready to kill his demonified brother. His stance terrified Kokushibu and his aura was the same as when he was in his prime demon slaying years. Without any time to react the old blind Yoriichi struck his brothers neck and cut it halfway. While holding his neck Kokushibu looked angry while he was waiting for the finishing blow, but that strike never came. His hate grew as he saw his brother had passed away from old age, with anger he slashed his brother's body in half to only see a flute, the flute that Kokushibu gave Yoriichi as a gift when they were children (Yoriichi said that the flute reminded him of his brother Michikatsu), kokushibu cried as he screamed that he hated Yoriichi.