Character A is a recovering blood drinking vampire. Vampires haven't come out to the world yet, so the invention of synthetic blood was to help them stay hidden. The problem is, is that he's not doing so well. One night he looses control and brutally attacks Character B and kills her. Plagued by guilt, he continues to return to the scene of the crime.
The twist is that she hasn't died and was actually turned and that's the real reason he keeps coming back to where he killed her. He's being drawn there since she keeps going back there too. Confused, scared and more then slightly bitter about the whole thing, she finally decides to step forward and reveal herself to him.
I'm thinking this will be a darker line. Maybe they can help each other with the addiction. Maybe they fight and make up on a regular basis. Maybe she hates him for what he's done, maybe he hates himself.