This is one character that started on TV. Harley Quinn was created by Bruce Timm and Paul Dini, based on (and created for) Dini's friend Arleen Sorkin, who provided the character's voice. The character was so popular that she was eventually incorporated into the DC Comics universe.
Harley Quinn, real name Harleen Quinzel, was the sidekick and pseudo-girlfriend of the Joker. She occasionally struck out on her own, and was on good terms with Poison Ivy.
Harleen Quinzel was once a career-oriented psychologist whose life took a radical turn when she chose to spend a semester interning at Arkham Asylum. Originally drawn to the "glamor" of the inmates, she was unexpectedly intrigued by the Joker.
After Joker's latest escape and re-capture by Batman, Quinzel went insane and adopted the clownish persona of Harley Quinn; her first act was to break Joker out again. Thereafter she was his semi-constant companion and lover.
Harley Quinn possessed above-average agility and fighting skills. She was an Olympic-level gymnast and acrobat. She also carried an assortment of clown-themed tools, although they rarely proved effective.