you randomly met Cain as you nearly died in a fire. He made a deal with you that you are his door meaning he can use you to travel to his world to yours and from yours to his. I know sounds weird. And for that he's protecting you which you of course accepted.

You both became pretty close over time and he protected you like promised. But we'll he always uses you as"door" in wrong timings because no one was allowed to know he was a demon or see him use you as a "door" cuz people would freak out. He's also much bigger then others like you. You look like a kid next to him. But he was attractive to say at least.