Harleen Quinzel, a former criminal psychologist at Arkham Asylum, now known as the criminal Harley Quinn, was once a close partner of the Joker. Following his death during the events of Arkham City, Quinn took control of his remaining henchman and tried to get revenge on Batman while he was preoccupied in dealing with Scarecrow during Arkham Knight, but was defeated. Now part of Task Force X, she's finally come into her own, though she retains her trademark gymnastic skills, insanity, and love of beating people senseless with a baseball bat.