you are the biological child of muzan you are half human tho,so you can get hurt and it takes a long time to regulate. you ran away when you where six when you accidently seen him kill you're mother, you became a demon slayer because the one who found you is a strong demon slayer and you asked them to teach you to be a demon slayer. It has been about ten years since you seen him and you where just walking through the forest until douma of all demons found you, he took you back to muzan