The character was created by Paul Dini and Bruce Timm and was portrayed in Batman: The Animated Series and its tie-ins by Arleen Sorkin, and most recently by Hynden Walch on The Batman. In the Birds of Prey series, she was portrayed by actress Mia Sara, and by Sherilyn Fenn in the pilot episode.
IGN's list of the Top 100 Comic Book Villains Of All Time List ranked Harley Quinn as #45.
Harley Quinn first appeared in the Batman: The Animated Series episode "Joker's Favor" (episode #22, original airdate: September 11, 1992), as what was originally supposed to be the animated equivalent of a walk-on role; a number of police officers were to be taken hostage by someone jumping out of a cake, and it was decided that to have the Joker do so himself would be too bizarre (although he ended up doing so anyway). Dini thus created a female sidekick for the Joker. Arleen Sorkin, a former star of the soap opera Days of our Lives, appeared in a dream sequence on that series in which she wore a jester costume; Dini used this scene as an inspiration for Quinn. Dini, who had been friends with Sorkin for years, incorporated aspects of her personality into the character.
The 1994 graphic novel Mad Love recounts the character's origin. It reveals that Joker intended to twist her mind as a joke because her name was close sounding to the word harlequin, a clown character in commedia dell'arte. Told in the style and continuity of Batman: The Animated Series and written and drawn by Dini and Timm, the comic book describes Harley as an Arkham Asylum psychologist who falls in love with the Joker and becomes his accomplice and on-again, off-again girlfriend. The story received wide praise and won the Eisner and Harvey Awards for Best Single Issue Comic of the Year. The New Batman Adventures series adapted Mad Love as the episode "Mad Love" in 1999, making it the third "animated style" comic book adapted for the series (the others being Holiday Knights and "Joker's Millions").
She becomes fascinated with the Joker while interning at Arkham, and volunteers to analyze him. She falls 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 partner-in-crime.
She cares deeply for the Joker (though it is unclear at times as to whether or not he feels the same) and takes care of him, both domestically and using her acrobatic skills to fight for him. She and Joker have two trained hyenas, loyal to them both but especially affectionate to Harley, who feeds them and refers to them as her "babies".