Dr Harleen Frances Quinzel is a complicated character - one who has operated on both sides of the law since she was first introduced to the world in Batman: The Animated Series back in 1992.
While she started life as a villain, and a lovesick foil for the Joker, she has grown, matured and changed a lot in her 32 years of publication. She's dumped the Joker entirely and, these days, is pretty firmly established as more of a heroic type. She's a core part of the current line-up of Birds of Prey while her own title focuses on her life and adventures with her love, Poison Ivy.
