From Slayer to Upper One
Kokushibo was Yoriichi's twin and a former Hashira who became a demon to surpass his brother through endless time.
Moon Breathing
His unique Moon Breathing style reflects his envy and his inability to match Sun Breathing, so he creates a dark mirror of it.
Tragic End
In his final moments, he confronts the futility of his choice—he never surpassed Yoriichi, only lost his humanity.