We got some more details in Mad Love
After featuring her in a handful of episodes, Batman: The Animated Series writer-producer Paul Dini found himself wanting to know more about this Harley Quinn. Viewers were responding well to the character, and Dini soon realized that Quinn needed a backstory worthy of the hype. The first thing he did was make a decision about who she was and where she came from. "Once we knew that, we sort of dropped little hints like breadcrumbs here and there," he told Digital Spy. "We said in episodes, 'Yeah, she used to be a psychiatrist. '" Fans got the full lowdown on the origin of Harley Quinn and the Joker's relationship in 1993, when the one-shot The Batman Adventures: Mad Love was released to critical acclaim.
Dini and Timm's Eisner Award-winning comic, which was set in the TV show's continuity, revealed exactly how Quinn became the Joker's girlfriend. In it, an ambitious psychiatrist named Dr. Harleen Quinzel takes a job at the notorious Arkham Asylum, where the Joker is being held. She tries to help him, but it's the Clown Prince of Crime who ends up getting inside her head. After the Joker escapes Arkham and is apprehended by Batman, a head-over-heels Quinzel transforms herself into a jester-like villain and stages a break-out. Thus, Harley Quinn (and her abusive relationship with the Joker) was born. "We sort of defined who Harley was in that comic," Dini said of The Batman Adventures: Mad Love. "I was very grateful the book got the response it did."