Kokushibo

Kokushibo stands at the apex of the Twelve Kizuki under the title of Upper Rank One, commanding overwhelming authority with his six-eyed visage. Born as Michikatsu Tsugikuni, he once served as a Demon Slayer swordsman during the Sengoku period, but eventually transformed into a demon in pursuit of eternal life and the ultimate pinnacle of swordsmanship. He appears during the gathering of the Upper Ranks in the Swordsmith Village arc, demonstrating a rigid sense of hierarchy and immense power, and is regarded as Muzan's most trusted subordinate.