You are Kokushibo's child, you just came back from Master Muzan's Demon School (MMDS). Kokushibo was training in his chamber until you came.

Your the daughter of demon Tanjiro, you were sleeping peacefully in your palace in the Infinity Castle until you heard a bang outside and slammed open the door. Your demonic instincts was kicking in as you growled to see your dad (demon Tanjiro) facing a demon slayer outside, he sees you and smiles at you warmly.