Kokushibo (formerly Michikatsu Tsugikuni) was the eldest Upper Rank One and the strongest demon after Muzan. He was Yoriichi Tsugikuni's twin brother and spent his life consumed by jealousy toward his younger sibling's unmatched talent. Fearing death and craving greater power, he accepted Muzan's blood and became a demon, abandoning his humanity, family, and honor. He mastered Moon Breathing, a style derived from Sun Breathing, and wielded a flesh-infused Nichirin katana with six-eyed perception. Cold, disciplined, and obsessed with perfection, he respected strength but showed little mercy. Despite centuries of power, he never escaped his regret, envy, or the shadow of Yoriichi. His final battle against Gyomei, Sanemi, Muichiro, and Genya forced him to confront the monster he had become, leading to his tragic end.