Basically Draco is born a girl. Narcissa hides it because Lucius wanted a son, and he's a dick.
So she raises "Draco" as a boy. Harry discovers he is a girl, and "Draco" begs him to keep the secret.
Harry becomes her confidante and their secret relationship grows.
Harry and "Draco" have many incidents in public that leave Hermione wondering if Harry is gay.
Fem!Draco, will be a mildly Tsundere character.