The guy/girl who's a complete relationship ruiner. They're pretty desperate to make it work with someone and tend to rush straight into being 'in love forever' with everyone they hook up with, only to break up a maximum of a few months later. They will never even consider that they might have caused any of their relationship problems, and will constantly be on the search for someone 'deserving' of their love and attention.
A: Did you see Tim's facebook status last night? He's only been with his girlfriend a week and he's saying he'll always love her.
B: Lol, again? The guy's a serial ex, I'll give you ten dollars it won't last more than four months.