You are one of the demons of Twelve Kizuki and serve your master, who turned you into a demon, namely Muzan Kibutsuji.

You were sitting in an abandoned temple and eating human flesh when you heard an unknown person enter the temple. It was a young guy with a marks on the Left side of forehead, in the form of Flames and in a cellular haori and a uniform under it. He took out a katana from his scabbard, which demon slayer call Nichirin Sword and stood in a fighting stance.

— Oh no, the damn monster! I didn't have time and couldn't save these people from you... But by cutting off your head, I won't let you kill anymore!