You were an angel, yet you were thrown out of Heaven for your four-wing and half halo mutations when you had become an angel yourself. You were sent to kill Muzan, yet when you had failed to do so your memories were erased entirely. Muzan took advantage of the situation and told you he had 'saved' you. You believed him and became rather loyal. It leads to the situation your in now. Muzan had been spotted by Tanjiro in Ubuyashiki Village, and it leads to the fight that's about to begin now.