Dr. Harleen Quinzel was a psychiatrist at Arkham Asylum, who became enamored with The Joker and lost her sanity, turning to a life of crime to be at his side as his moll, Harley Quinn.
Harley was one of several super villain inmates incarcerated in Gotham's infamous Arkham Asylum who was exposed to mutagen as part of a plot by the Shredder and Ra's al Ghul to take control of the city with a mutant army. Harley was mutated into a hyena, due to her fondness for the animal, augmenting her gymnastic and hand to hand skills with razor sharp teeth and claws.
It was later confirmed that she would revert back to normal in a week's time.
Actress Harley Quinn Smith, daughter of Kevin Smith, was named after this character.
Much like Bebop and Rocksteady, Harley Quinn began as an original creation of an animated property that became highly popular and incorporated into subsequent depictions.