Harley Quinn (Batman: Arkham City)

An Arkham Asylum psychiatrist assigned to treat the Joker, Dr. Harleen 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. She remained at the Joker's side during his painful recovery from the effects of Titan.

Batman: Arkham Asylum

 * See: Harley Quinn (Batman: Arkham Asylum)

After Arkham
Six Month's later, Harley has been incarcerated in a high security cell, believing the rampant rumors that the Joker is dying to be false. However her hopes are soon shattered when she overhears a conversation between some guards not only confirming the Joker's short lifespan but also a conspiracy to have him killed. With this she kills and replaces one of the guards planning to kill Joker and rescues him right in the middle of the attempt on his life.

With this she hijacks a supply boat and uses it to sneak into the recently completed Arkham City, setting up her and Joker's base of operations in a abandoned factory. According to Hugo Strange, Joker's inevitable fate is having a hard impact on her causing her to fall into a depression and adopting a increasing millitant approach. Later on she is shown helping Joker with his recruitment campaign screening a potential recruit Lester Kurtz, giving him his clown mask, and showing him to his room before he ran off.

Batman: Arkham City
Harley Quinn has been confirmed to appear in the sequel. Her style has been redesigned. She is more vicious and deadly as a result of Joker's slow and painful death, she's also has yet another new outfit, a combination of her original, a gothic and a biker chic outfits, with a increase focus on her sex appeal (Dr. Hugo Strange theorized this to be a manifestation of her extreme manic depression as a result of Joker's impending death). At one point in the first gameplay trailer, Harley Quinn, before attacking Batman, is considering not attacking him, as she doesn't want his blood to ruin her new outfit. She is the leader of the Joker's Army and carries out his errands while he is still recovering.

In the hour long demo for Batman: Arkham City, Harley Quinn & her troops take over the Medical Center and kidnaps a doctor.

Tara Strong will be voicing her in the game, taking the regular Harley Quinn actor, Arleen Sorkin's, place.

Arkham City Profile: Dr. Hugo Strange
Harleen Quinzel

A.K.A Harley Quinn

The Joker's plaything and sometimes girlfriend, Harley Quinn was happy to leave Arkham Asylum so long as she could be with her 'puddin.' Her devotion to the Joker is admirable, but it is clear that his illness has laid a heavy burden on her as well. Her behavior has become evermore erratic and vicious, as she appears to be in a state of emotional metamorphosis, manifested by her change in wardrobe to a more militant, harsh appearance.

Dr. Hugo Strange: Side Note

Ms. Quinn’s behavior has become evermore erratic and vicious.

~ Dr. Hugo Strange

Trivia

 * In one of the Arkham City preorder commercials for GameStop, Harley is the narrator.