Webmuichiro's red blade by itself didn't cause kokushibo to crumble, it was just the slowest part of his body to heal after getting attacked. The dude even healed his head after getting. Webin the current time, he is the ancestor of muichiro tokito, mist breathing hashira of the demon slayer corps. In his human life, kokushibo was jealous of his. Webkokushibo sees the muscles, ribs, and blood flow of muichiro identical to how tanjiro saw those of akaza and giyu and determines that muichiro is of similar blood to him. Webgyomei was assisted by muichiro, who is actually a descendant of kokushibo, and a handful of other fighters. It was in his moment of death that kokushibo. Webafter telling kokushibo his name, muichiro is stunned when he is told that he is a descendant of his. The aura given off by kokushibo is intimidating to the point that. Webso the two are related with muichiro being kokushibo's descendant. I read through the manga but don't actually remember how that came to light or how they figured it out.
