Yoriichi Tsugikuni is a Demon Slayer and the most powerful Demon Slayer. He is also the inventor of Breathing Styles, being the one who created the first ever Breathing Style, Sun Breathing, that would eventually become the Hinokami Kagura used by Tanjiro Kamado and members of the Kamado Family and the five fundamental styles taught to all Demon Slayers. He is the younger twin brother of Michikatsu Tsugikuni, who later became a demon and joined the Twelve Kizuki now as Kokushibo.