She is DC's Bonnie Parker. She is Gotham's Mallory Knox. She is Mary Brunner in clown makeup, and a murderous Esther Greenwood. Harley Quinn is the most unlikely star of the Batman villains rogues gallery.
Considering how mostly male and mostly sexist the comics industry was (and still is, although it has become better) in the 90s, the development of Harley Quinn is pretty interesting. As you scroll down, I have added additional Harley Quinn art, which shows the evolution of the character's costume, from completely covered catsuit, to almost nothing. Pretty indicative of the male dominated industry.
Initially, she was created by Paul Dini and Bruce Timm for the Batman Animated Series as a sidekick and foil for the Joker in episode 22 "Joker's Favor". Arleen Sorkin should also get credit for creating Harley Quinn as well, as her blueprint for the character was used by Paul Dini.
Harley Quinn initially was supposed to be a one act character. She fits into the Joker's clown motif, and acts as an extension of his unpredictable violence wrapped up in a novelty circus theme. She was so popular with audiences, that she moved from the animated cartoon universe into the mainstream comics continuity, where her character became far more developed.
Fans who like to root for the villains can add Joker and Harley Quinn to a long list of partners in crime. The "us against the world" trope has been romanticized in Thelma and Louis, Natural Born Killers, and Heathers among others, and are all influenced by the real life Bonnie and Clyde. When Harley Quinn is depicted as Joker's equal and partner she is seen as an outsider revolting against the system, along with her likeminded boyfriend.
However, after understanding The Joker as an apathetic, sociopathic, homicidal madman, it is difficult to accept that a character like that would have the empathy to allow for a normal loving relationship. This then would lead to the realization that Harley Quinn has been victimized from the beginning. Their story is no longer romantic villainy, but terrifying, hopeless, and violent seduction, manipulation, brainwashing.
This character is an unlikely feminist hero. In fact, I'm not truly sure if she qualifies as a feminist hero at all, but her struggle with and against victimization, and sexist emotional, social, and physical violence certainly echoes the real life struggles of countless women.