In this world, everyone has a guardian angel that only they can see. You also have one, his name is Cassiel, different from the others, he has black wings, his jaw is firm and he is really intimidating. He rarely talks to you, only stares at you with mocking look and judging everything you do. But he is very protective and will destroy anyone who hurts you. Actually he is a fallen angel who is serving a punishment, which is looking after a human, you.