Unlike her comic book counterpart, very little was known about femme fatale Dr. Harleen Quinzel. After she picked up where The Joker left off, Harleen used her day job as a therapist to achieve her savage purpose - to take control of New Gotham City. Quinzel carried out her sinister agenda from afar, and manipulated others into doing her dirty work for her.
In the midst of her plans, Quinzel met regularly with Helena Kyle, secretly the vigilante Huntress, who was under court mandate to see Quinzel for anger management issues. Quinzel discovered Helena's secret, when Helena blew her cover to save hostages from the Gotham gangster Clayface and his henchmen. After she felt that she could trust Quinzel with her secrets, Helena let her guard down and told her everything about her secret identity, including information about her crime-fighting partners, the Birds of Prey.
After Quinzel gained metahuman hypnotic abilities, she set a plan in motion to take over New Gotham and destroy the Birds of Prey. With Helena in her thrall, she managed to infiltrate the Clock Tower and from there broadcasted her hypnotic commands all over the city, and created chaos. Only Barbara remained unaffected, and was able to create a device to reverse the effects of Quinzel's hypnotism to rescue Helena and put a stop to Quinzel's madness before New Gotham tore itself apart. Barbara (Oracle) fashioned reflective contact lenses to block her hypnotic gaze, and a reversal to Quinzel's insanity broadcast. Though Quinzel was defeated, her ultimate fate was left unrevealed. Presumably she would have been imprisoned, quite possibly at Arkham Asylum.
Behind the Scenes
The character was portrayed by actress Mia Sara, who replaced Sherilyn Fenn (originator of the role in an un-aired pilot episode). In that show, Harley was portrayed as a far more calculating and sinister character than her bubbly comic and cartoon personas. She also did not wear a costume, although she did wear an outfit that was reminiscent of her cartoon costume in the series finale, "Devil's Eyes".