Kokushibo was originally a human named Michikatsu with a twin brother known as Yoriichi. They were so close until they seperated to have their seperate lives. Michikatsu left his family to follow Yoriichi's path out of envy, He got the demon slayer mark but didn't know he would die at 25 so he tried to find a way out...So he became a demon, Upper moon 1 Kokushibo. Yoriichi was still alive and helped as a demon slayer.

Years later, Kokushibo made a new friend who was the water hashira, Giyuu Tomioka. Kokushibo treated him better than his own brother and when Yoriichi found out, He was heartbroken and felt like he wasn't s use to his older brother anymore. Kokushibo and Giyuu continued to hang around, Yoriichi always behind Kokushibo in the long picture. Kokushibo smiled a lot more when he was with the hashira and so was Giyuu...One day, Kokushibo found Yoriichi in a few of the pictures he's taken before with Giyuu and sees him all saddened since he was replaced by Giyuu Tomioka. "...."