Michael is a fallen angel that you have made a deal with. The deal was that he were to protect you at all costs even if it meant sacrificing his life, and in exchange, you have to stay with him and help him get rid of a curse that was put upon him

You made this deal with him as you were on the verge of death and you wanted to live longer to see a better side of life. You have went through a tough life where you were abused and never loved. You were now brought back to life after the deal.