So many times in real life we see a girl like Harley that falls for the "Bad Boy" on the block. Why is this? I think that is why she is one of my favorite good girl gone bad characters. Every time I see her with The Joker, I catch myself thinking, "Don't do it, Harley!"
We all know how this crazy love affair happens when she becomes fascinated with the Joker while interning at Arkham, and volunteers to analyze him. She fell hopelessly in love nearly instantly with the Joker during their sessions, and she helped him escape from the asylum more than once. When the Joker is returned to Arkham in battered and broken state after a battle with Batman, the sight of the injured Joker drives Harley to don the costume and become Harley Quinn, the Joker's sidekick and love.