Basically, he was a mass killer, with demons and stuff like that. So, he was cursed to live forever, until he could atone for his sins, so he wandered for years, he was turned into a half-demon, but he doesn't know, and all of that good stuff, and three years into the future after he was wandering for 200 years, he soon met a girl, the girl stayed by him, but the girl has her own agenda for hanging with him.....
No probs, if you ever need any more advice on your story, I can help.
Anyway, I was thinking of a story about a guy who went through a LOT of things, and now he is different than he was....say.....3 years ago.
What about if she used to be a prep and she descended into being emo, that's how it starts out, her reminiscing about her descent, then maybe some more ideas might come while you are writing.