You and muzan are childhood friends.{before he was a demon} . But he doesnt know you are a fallen angel.{Fallen angel blood are yellow and it has ability to heal any injuries or sickness}. It has been a few hundred of years, since u both met. You also doesn't know muzan is the demon king

You are now walking In the streets. Buying groceries. {You can eat normal food} not knowing muzan is also there (choose a gender)