First appearance: Batman: The Animated Series
episode "Joker's Favor"
Harleen Quinzel received her PhD in Psychology and Neurological disorders from Gotham University. Talented, intelligent, and beautiful, her path to success seemed clear and she knew it. This self assured arrogance would eventually lead to her downfall.
She was attracted to the challenge of working with extreme personalities. So, she managed to get herself a job at Arkham Asylum in Gotham City, home of some of the most extreme personalities available. One such inmate was the notorious Joker.
After studying his case file for months, she thought she was prepared for any trick he could toss at her. That is except for his stories of an abusive father and runaway mother. She began to feel sympathy for him, and eventually love.
She was convinced that Joker was a sad child lost in a world that just didn’t get the joke. And the main obstacle between herself and the Joker’s sanity was the self righteous Batman.
So she did what any one would do. She put on a costume, gassed the orderlies, and broke the Joker out of Arkham.
She became Harley Quinn. The Joker’s completely loyal henchmen and love-muffin. Though he is dismissive, abusive, and will rat her out whenever it suits him, Harley still believes that the Joker shares her love. And one day when Batman is no more, there will be nothing more to stop her and Puddin’s happiness.
Harley Quinn’s character on the Batman: The Animated Series was voiced by the very talented Arleen Sorkin.
During DC's history, several other women have used the harlequin as their alias. The original Harlequin was Molly Mayne-Scott, who first appeared in All-American Comics #89 (September 1947): “The Harlequin”.
The second was actually Joker’s daughter Duela Dent, first appearing in Batman Family #6/2 (July-August 1976): "The Joker's Daughter". And as Harlequin in Teen Titans Vol. 1 #49 (August 1977): "Raid of the Rocket-Rollers".
The third was Marcie Cooper, first appearing in Infinity, Inc. #14 (May 1985): "Concert In the Key of... Chroma". And as Harlequin in Infinity, Inc. #46 (January 1988): "Swamped".
The primary source for the bio, was Batman: Gotham Knights
episode #24 “Mad Love”
says: Okay, according to my DC Comics Encyclopedia, Harley Quinn made her comics debut with "Batman: Harley Quinn #1" in October 1999. The book is far from complete and doesn't even mention the earlier Harlequins.