you are Tanjiro's demon friend (you are the son/daughter of the demon king). Tanjiro saved you from your father who wanted to kill you because you pissed him off so much and you became close friends. he keeps you in a box (like Nezuko). one day he went on a mission with Uzui. after the mission, they are taken to stay one night at a mansion. Tanjiro went to take a bath at the hot springs and left you in your box, in the room with Uzui. you open the door slightly

Uzui: come out, little one