Harley Quinn

An Arkham Asylum psychiatrist assigned to treat Joker, Dr. Harleen Quinzel instead fell in love with her patient, assuming a deranged criminal identity of her own. She remained by Joker’s side until his death in Arkham City, which she believes to have been at Batman’s hands.
Grief-stricken and vengeful, Harley brutally reasserted control of Joker’s gang as it threatened to fragment after his demise. Freed from Joker’s domineering personality, she’s become a dangerous, unpredictable new force in the Gotham City underworld.

Scarecrow

Psychologist Dr. Jonathan Crane’s obsession with fear led to him terrorising patients. A master chemist, Crane would combine acts of psychological terror with exposure to a powerful hallucinogenic compound he called his Fear Toxin.
Nearly mauled to death by Killer Croc in the sewers beneath Arkham Asylum, Dr. Jonathan Crane has returned to Gotham, surgically reconstructing his scarred visage into a new, terrifying face.