Muzan Kibutsuji

Kibutsuji Muzan is a Demon, the first of his kind, and the progenitor of most other Demons.

Asakusa Arc
Traveling through a crowd in Asakusa with his family, Muzan is accosted by an angry Kamado Tanjirou. His daughter asked him who the young Demon Hunter was, and he asked the youth if there was something he wanted from him. His wife then approached them, asking if Tanjirou was somebody familiar to their family. He denied this, wondering if the youth had mistaken them for somebody else. He then quickly slashed a passing man, turning him into a Demon.