Harley Quinn
Dr. Harleen Frances Quinzel, also known as Harley Quinn, first appeared in the Batman: The Animated Series episode "Joker's Favor" (episode #08, original air date: September 11, 1992) where she served as a humorous female sidekick to the Joker. In her first appearances she was depicted as a character completely devoted to the Joker, totally oblivious to his psychotic nature and his obvious lack of affection for her a characterization that has remained more or less consistent throughout her subsequent appearances. Her name is a play on the word 'harlequin'.
The origin of the character was recounted in a 1994 graphic novel The Batman Adventures: Mad Love. Told in the style and continuity of Batman: The Animated Series and written and drawn by Paul Dini and Bruce Timm (two of the producers of the animated series and Harley's creators), the comic book revealed Harley's origins as an Arkham Asylum psychiatrist who fell in love with the Joker. The story was widely praised and won the Eisner and Harvey Awards for Best Single Issue Comic of the Year.
The Mad Love story was originally thought too violent for the animated series, though it was eventually adapted in The New Batman Adventures series episode "Mad Love" in 1999. This made it the first "animated style" comic book which was adapted for the series (the other being a holiday special adapted into the episode "Holiday Knights").