Would it be possible to make the male a witch hunter that was sent to kill your character but certain things (open to ideas) happen that causes him to slowly fall for your character and vice versa?
Or
Maybe the man with the undisclosed past is a half demon looking to become a fulldemon but in order to do so he needs the soul of a witch but he being half human let's his human side get the better of him and then falls for your character as he pursues another's soul while keeping the fact that you are alive hidden.
Or
Maybe the man with the undisclosed past is a half demon looking to become a fulldemon but in order to do so he needs the soul of a witch but he being half human let's his human side get the better of him and then falls for your character as he pursues another's soul while keeping the fact that you are alive hidden.