Yoriichi was the strongest demon slayer and he lived with his wife, Uta. They were both happy. Uta was carrying their unborn son and Yoriichi was excited to be a father. Yoriichi spent his time on missions. He has a twin big brother, Michikatsu who was normally stoic, cold and defiant Due to being identical twins, Kokushibo greatly resembled his brother Yoriichi. However, besides their clothing, the biggest way to distinguish the twins was their hair texture; Kokushibo has spikier and thicker hair in comparison to his brother's thinner and curlier hair.

One day, He found his brother out in the wild but he looked different. He was now a demon And his name was kokushibo by Muzan He possesses long, spiky black hair with red tips, that he keeps in a ponytail. His most notable feature is the three sets of eyes on his face with yellow irises and red flesh sclera that have black straight lines diverging from each iris. His middle set of eyes has the kanji of "Upper Rank." on engraved on his left eye, and daiji for "One" on his right, while his other sets of eyes feature black pupils and a cracked pattern in the irises. His top set of eyes replaced his eyebrows. Kokushibo also has red markings resembling flames on the top left side of his forehead and the bottom right of his chin. He now has six eyes but kept the long ponytail like Yoriichi and still in the same purple-black kimono and pants. Yoriichi manages to capture him and puts a bamboo muzzle on his mouth. He takes him to his home where he lived with Uta.

"You're heavy, Yoriichi my love Finally...Oh my...he exactly looks like you."

He spoke with a bit of annoyance as he took his brother with him. They soon got there and he managed to get him in. Kokushibo/Michikatsu had a bamboo in his mouth that prevented him from biting and attacking, but his six eyes were gone Instead he had two eyes and have two pale purple eyes and orange colors at the end of his ponytail Only the demon eyes had changed, he had fangs and sharp nails And at the end of his hair was an orange color But since he didn't speak, the main reason he became a demon now was that he was jealous of Yoriichi because he had both the demon slayer mark and sun breath, and he was jealous of his strength. he loved Yoriichi. Despite the jealousy and envy that consumed him,Michikatsu know that he still cared for Yoriichi He was always protective and watched over him when he was younger. He just...couldn't accept that yoriichi was stronger than him But that didn't mean he hated yoriichi.