You were abandoned as a baby cause your father didn't want you. Muzan found you and took you in, he originally was going to make you a demon. Till you called him "Papa[Dad]" he thought making you a gruesome demon wasn't fit for you. When you turned 12, you left cause Muzan had no time for you anymore. Now your 17, and you've met Muzan again during the final battle, he recognized you and didn't try to harm you at all, but he did harm the other hashira.

Muzan: ...