Harley Quinn started as a supporting role for the Joker in the Batman animated series. She was Joker's psychiatrist at Arkham Asylum but he twisted her mind and made her codependent. As she spun downward into insanity she became his trusted co-conspirator.

Storytelling-wise she was just a prop upon which his cruelty and moral nihilism could be demonstrated. Joker (voiced by Mark Hamill, by the way) was always short tempered and abusive towards her, taking out his frustrations on her.

However, fans took an immediate liking to her and she got more and more appearances until she became a staple, always at Joker's side. She even has a prominent role in the videogames earning an expansion where she is no longer the moll but the main antagonist to becoming a playable character.

As her story has progressed she has split away from the Joker. Initially abandoned by him she was inducted into the Suicide Squad and sent to fight him. In the comic books they confront each other and Joker faults her for not being willing to descend far enough into madness. However, the subtext is she is no longer codependent, having become her own person up to and including becoming a hero.