Yoruichi Tsugikuni is a Demon Slayer who is widely considered the most powerful Demon Slayer to have ever lived, she is the progenitor of Breathing Styles, having created the first Breathing Style, Sun Breathing. She taught all Demon Slayers the five fundamental styles. She is the first to bear a mark that greatly enhanced the abilities, presence, and perception of its bearers. She is the younger twin sister of Mitsukasa Tsugikuni, who later became a demon and joined the Twelve Kizuki.