Harley Quinn (Harley Quinn's Revenge)

An Arkham Asylum psychiatrist assigned to treat the Joker, Dr. Harleen Frances Quinzel instead became obsessively fixated on her patient, believing herself to be in love with him. She helped him escape confinement and took on her own criminal identity as Harley Quinn, named after the french court jester Harlequin, and would become almost as deadly as her boyfriend to the Dark Knight and Gotham City. Over the years, she participated in the Clown Prince of Crime's various schemes for destruction with her only motive being the approval and love of the Joker, though the villain rarely ever reciprocated her feelings in the slightest.

After the failure of one of Joker's more deadly and recent plans, culminating in a massive Arkham riot and breakout, Joker became deathly ill following an overdose of Titan, a unique steroid compound that was being developed at the asylum and was pivotal to the Joker's designs. Harley remained at his side during his painful recovery from the effects of Titan in Arkham City, aiding him in what would later become his last and most twisted joke on Batman and Gotham. After the Joker's untimely demise, Quinn took complete control over the remains of his gang in her own quest for vengeance against Batman and all of Gotham in honor of the Joker.

Story
Immediately following Joker's death, Harley regrouped the clown's gang and staged a trap for officers arriving at Arkham City for payback for Joker's death, but was quickly knocked out by officers and taken into custody. While at a psychiatric ward as Arkham Asylum was being prepared to be reopened, Harley managed to escape and stole explosives from the GCPD, and reunited Joker's gang at the Industrial District in the ruins of Arkham.

Ambushing SWAT forces that had been sweeping through the city to find any more thugs to evacuate to Blackgate Prison, Quinn's forces kidnapped two police officers, and kept the district under lockdown from the GCPD, forcing Commissioner Gordon to stage a stakeout on the area. Summoning Batman, Gordon explained about the hostages and Batman enters the district to rescue them, and makes his way to the Steel Mill to find one of two cops. Discovering Harley had a secret base in the shipyard, the Dark Knight interrogates her thugs for information on the base and eventually finds an entrance, as the second cop remains inside.

When the Caped Crusader arrived and rescued two of the officers, Quinn wounded Batman and locked him in an air-tight glass ball in a giant memorial for her beloved. Two days later, Robin arrived and began making his way at the docks and finds Batman's utility belt. Traveling deeper into the docks, Robin eventually comes face-to-face with Batman in the giant Joker shrine, and fights Harley and her henchmen to retrieve her key card to unlocking the ball keeping Batman captive. Returning to the shrine, Quinn unleashes a Titan-enhanced thug on Robin, who defeats it and frees the Caped Crusader. Harley then activates the next step in her plan; rigging the area with GCPD-stolen bombs, forcing Batman to hunt them down while Robin rescues the other police officers.

Realizing she's close to defeat, Quinn calls Batman back to the shrine where her final plan begins- activating the Wonder City mechanical guardians (stolen by the Joker during Protocol Ten) and watching from the ten-foot Joker structure the fight ensuing. Destroying the robots, the Dark Knight then heads for Harley Quinn, who, in her last, final step of revenge activates the bomb hidden in the Joker structure, planning to kill both Batman, Robin and even herself, to reunite her with her beloved.

Batman is able to get Quinn and himself to safety, but soon fears that Robin was caught in the explosion. Harley relishes in Batman's anguish and moves to stab him in the heart to finish her revenge, only to be knocked out cold by one of Robin's projectiles as the young hero reveals himself to be alive and well. Harley Quinn is once again arrested by Gordon while Batman leaves, still scarred from Talia and Joker's death, but Robin assures Gordon that Batman will be fine. With Harley's plans ruined, her gang is rounded up by the Gotham Police and Arkham City is finally, officially shut down for good.