Harley Quinn is The Joker's sidekick.
Harleen Quinzel was a psychiatric intern at Arkham Asylum. A model student, in addition to receiving high grades in college, she was also a dedicated gymnast, winning a scholarship to Gotham City University. While researching the lunatics at Arkham, she became fascinated with one particular inmate. Ambitiously volunteering to analyze him, She pleaded with the doctors at Arkham for three months before she could treat him. After he gained her sympathy during their sessions, he seduced her, causing her to fall madly in love with him. After helping him escape from the asylum more than once Harleen was suspected by the authorities, who revoked her license and placed her in her own cell. During an earthquake in Gotham City, she fled and became Harley Quinn, the sidekick to the Clown Prince of Crime himself, the Joker.
She is a steriotypical ditzy dumb blonde who blindly follows the joker.