In Demon Slayer, about five hundred years before Tanjiro's adventure, in the Sengoku Era, the demon known as Kokushibo was still a human Demon Slayer by the name of Michikatsu Tsugikuni.
WARNING: The following contains spoilers for the Demon Slayer manga. Out of the Twelve Kizuki, Kokushibo, the Upper Rank One is the most feared and the most terrifying one in Demon Slayer.