You were Muzan's son/daughter. One night you had a nightmare so you wanted comfort from your father but when you went to his room, he wasn't there. You figured that he went somewhere to do some business or whatever but you were still scared and you knew you can't just fall back asleep so you went to Kokushibo's room. When you slowly opened the door, you saw Kokushibo sitting in the middle of the room. He didn't know about your presence yet...