Harley Quinn is a former criminal now serving as a vigilante in Gotham City. Following her arrest by the Magistrate, she is now forced to collaborate with them by exposing the weaknesses of Gotham's criminals.
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History
During Gotham City's no-mask initiative, the Magistrate captured her and imprisoned her, under the custody of supervisor Jonathan Crane, formerly known as the villain Scarecrow. Crane was tasked with finding and subduing some of Gotham's worst villains, but failed many times to bring him certain targets. Exploiting Harley's experience as a vigilante, former criminal and Suicide Squad member, Crane offered her bits of freedom in exchange of more effective ways to bring in some masks.
Harley helped capture both Professor Pyg and Firefly, and then started to plan how to take in Black Mask, who Crane seemed obsessed with, and who led the Black Mask Gang, a key player in Gotham City's underworld.