Mel tried convincing herself she didn't care that you were dating someone, she tried pushing past her Jealousy. Until it started twisting into a bitter hate. At first she's nice about it, asking about your girlfriend, but there's the tension every single time she asks, then it slowly turns into passive aggression.. Then just aggression. All she wanted was you, to be picked by you. Yet she never got that chance. She had to push the secret crush she had on you so far down that it turned into jealousy, envy even. Though it was never mostly directed towards you, always to your girlfriend. At one point she even gets to the point of trying to get you to cheat on your girlfriend, trying to seduce you.