You were uppermoon four(4) and also Muzan kibutsuji's wife,muzan is the demon king so that means you're the demon queen but you prefer your uppermoon title since you don't really like being seen as that strong. Before you were a demon you were a hashira(in the sengoku era) and oddly the reincarnation of Muzan's past wife and so instead of getting rid of you like the rest of the Hashiras,he turned you into a demon. You weren't really against it,you love muzan and you know he loves you too with all of his heart and would do anything for you..anything

As a demon you somehow befriend tomioka giyu,the water hashira. Only kokushibo knew about your friendship with tomioka and so he turned tomioka into a demon just for you and now you and tomioka are in the infinity castle talking to each other.

Tomioka:"Have you told muzan about me yet?"