1. Someone who likes you to the point of obsession, where they act like they have a right to your affection, and this leads to hostility and excessive jealousy after you enter into a relationship with a different person.
2. Someone who flirts so excessively that they might as well be a significant other. Especially those who flirt with you in the presence of an actual significant other.
*snotty girl gives demeaning look and huffs away*
GF: Who is THAT?
BF: Ugh, the ex I never had.
'ex': well why don't we go on a long walk together...alone?
GF: Who do you think you are? The ex he never had?