Dr. Harleen Francis Quinzel, M.D. is an Arkham Asylum psychiatrist who becomes fascinated with the Joker while interning at Arkham and volunteers to analyze him. She falls hopelessly in love with the Joker during their sessions, and she helps him escape from the asylum more than once. When the Joker is returned to Arkham after a battle with Batman, the sight of her badly injured patient drives Harleen insane, leading her to quit her psychiatrist job and don a jester costume to become Harley Quinn, the Joker’s sidekick. She later becomes friends with Poison Ivy, who injects her with an antitoxin which gives her increased agility and an immunity to toxins.