Kokushibo is a demon affiliated with the Twelve Kizuki, holding the highest position, Upper-Rank One. Nearly five centuries ago during the Sengoku Era, Kokushibo was a human by the name of Michikatsu Tsugikuni, a former Demon Slayer and the older twin brother of Yoriichi Tsugikuni. He is also the ancestor of Muichiro Tokito and Yuichiro Tokito and the one responsible for turning Zenitsu Agatsuma's senior, Kaigaku into a demon and joining the Twelve Kizuki. Demon Slayer anime cursor Kokushibo.
