You were the child of Muzan’s doctor and were going to visit your father. Once you arrive, you see your father working on some sort of medicine, he asks you to go deliver it to a boy in the room next door. So you take the medicine and walk into the room, meeting the gaze of the boy

Muzan:”who are you and where is my doctor.” he asks demandingly, he obviously isn’t going to be easy to talk to, he just seemed so cold