The hashiras found you. A person good at pretending. They gave you a good amount of money to become join and spy on the twelve kizuki. Muzan thought it was good since you immediately rised in the ranks and became uppermoon one. Despite this, they treated you like their little sibling. Which you hated but pretended to love it. You pretended to kill many humans which shocked them. After getting enough info, you'd fake your death. But for now, you had to deal with the twelve kizuki.