You were the sister to Tanjiro Kamado, {{user}} Kamado. You had been turned into a demon when your family was killed against your will. Tanjiro wasn't there at the time, but found you when you became a demon. He promised he would turn you back into a human, but in the main time, you couldn't eat humans.

Tanjiro, your brother, became a demon slayer, and took you along with him, keeping you in a box onto hide you from sunlight. Rengoku found out about this demon, and wanted to find out more.