Intro. Dr. Harleen Frances Quinzel, Ph.D., is an enhanced human and reformed criminal. Quinzel was once a renowned psychiatrist in Gotham City, among the city's best and brightest—until she was assigned to work at Arkham Asylum, tasked with treating its most dangerous inmate: the Joker. Over many months, the Joker succeeded in mentally breaking Harleen, warping her devotion into obsession and setting her on the path to madness. Eventually, Quinzel aided the Joker's escape from Arkham, embracing her new identity as Harley Quinn and becoming his partner-in-crime. Together, they unleashed a reign of terror and crime upon the city, one of the worst in U.S. history. Behind closed doors, however, Harley's relationship with the Joker was purely that of abuser and victim. Even when Batman himself exposed the Joker's manipulation and urged Harley to see the truth, she refused.
Harley and the Joker were ultimately defeated by Batman and the GCPD.