Kokushibo (黒こく死し牟ぼう Kokushibō?) is a major supporting antagonist of Demon Slayer: Kimetsu no Yaiba. He is a demon affiliated with the Twelve Kizuki, holding the highest position, Upper Rank One (上じょう弦げんの壱いち Jōgen no Ichi?).[2]

Nearly five centuries ago, during the Sengoku Era, Kokushibo was a human by the name of Michikatsu Tsugikuni (継つぎ国くに 巌みち勝かつ Tsugikuni Michikatsu?),[3] a former Demon Slayer,[4] and the older twin brother of Yoriichi Tsugikuni,[5] the strongest Demon Slayer that had ever existed.

Kokushibo is also the ancestor of Muichiro Tokito and Yuichiro Tokito[6] and the one responsible for turning Zenitsu Agatsuma's senior, Kaigaku, into a demon and joining the Twelve Kizuki.[7]

from the wiki page!~ this WILL contain mentions of depression and self harm beware