Can I just say, Harleen Quinzel is just about my favorite Batman villain. I mean, she’s throwing diamonds like confetti because she’s happy; seriously, how cute is that?
So Harleen Quinzel, M.D. is a psychiatrist who volunteers to analyze the Joker in Arkham Asylum because he catches her eye.
She falls head over juggling pins for the Joker after a particularly touching backstory session (that the Joker may or may not have fabricated on the spot) and so, naturally, she helps him escape Arkham.
When he comes back all injured and stuff having lost in a fight with the Bat, Harleen looses her mind.
She quits her psychiatry career to put on the mask and the jester costume, changing her name from Harleen Quinzel to Harley Quinn [as in harlequin] in order to cheer him up and ultimately in hopes of gaining his affections.
Which kinda half works…
When she befriends fellow female antagonist Poison Ivy, she gains super strength, agility, and an immunity to toxins after Ivy hits her with an antitoxin.
Harley Quinn was created for the Animated Series for plotline purposes since in the “Joker’s Favor” police officers are taken hostage by someone jumping out of a cake, and it would apparently be ridiculous to have the Joker jump out of the cake himself [which he does anyway] so they fashioned Harley Quinn as his sidekick.
Although she was created for the sake of the TV show, the comic books adopted Harley as the Joker’s on-off sidekick. As with the Joker, the comic version of Harley Quinn is much more dangerous and violent and less humorous and quirky than the animated version. However, unlike the other villains Batman faces, Harley Quinn does exhibit mercy and compassion from time to time in both the comics and the animated series.
Harley Quinn had her own comic series, dealing with her solo act. She evenually starts a gang and then flees Gotham, heading to Metropolis with Poison Ivy. The series ends with Harley checking herself into Arkham, finally understanding that she needs help.
In The New 52 event, Harley is fully revamped—skimpier costume, bleached skin, altered hair, new origin. She goes on a murderous frenzy following a fall out with the Joker. Black Canary shuts her down, and Harley is forcibly inducted into the Suicide Squad ((featured in The Brave and the Bold (vol 1) #25, Legends #3, Secret Origins (vol 2)#24)) but when she hears a rumor that the Joker is dead, she betrays the Suicide Squad and turns herself in to the Gotham Police Department in an attempt to attain the skinned face of the Joker (happy comics, here) which she then places over Deadshot’s face ((Deadshot, by the way, decided to join the Suicide Squad primarily because of a desire to die in a spectacular fashion)) so she can carry on a conversation with the Joker.
She’s only been in a few of the movies, a planned Batman film ((Batman Triumphant)) and Batman Beyond: Return of the Joker.