Jennings was born with a crippled arm. Her father, an alcoholic, was abusive towards her. One day she fought back, pushing him down some stairs, resulting in a broken neck. She was charged with murder and sent to Blackgate Penitentiary even though she did it in self-defense. While at Blackgate, she was approached by two men who told her that they could fix her crippled arm, and make it better. She then decided to take part in Wayne Enterprises' program at Pinewood Farms where they experiment on her crippled arm and mutated it into a reptilian claw. When Thomas Wayne caught wind of the experiments occurring at Pinewood, he shut the program down and paid to put the surviving test subjects, including Jennings, into hiding. From then on, Thomas formed a sort of father-daughter bond with Jennings. He visited her on a regular basis, spent time with her, bought her gifts such as a music box for her birthday.
Two months prior to the murder of Thomas and his wife Martha, Thomas went to visit Karen following a break in at his home Wayne Manor. During the visit they were attacked by an assassin, but Karen managed to kill the man by slashing his throat with her clawed reptilian hand. Thomas then buried the man's body in the woods as he didn't want to alert the authorities to Karen's location.
Helping Bruce Wayne
Some years later, Bruce Wayne and Alfred Pennyworth find where Karen Jennings has been living ever since Pinewood. Though she doesn't know the names of the people involved, she does take them to where Pinewood Farms is located. While there, the three of them are ambushed by two men sent by Hugo Strange, one of whom Karen slashes across the throat, while Alfred incapacitates the other with a gunshot. However, the three of them are apprehended by the police, who are surprised at the sight of Karen's reptilian arm. After interrogating Bruce Wayne about his and Alfred's actions, Nathaniel Barnes has Karen prepped to be returned to Blackgate Penitentiary. Upon finding out from Ethel Peabody that Karen Jennings is being returned to Blackgate Penitentiary, Hugo Strange sends Victor Fries to silence her. Bruce tells Jim Gordon what happened. Gordon, Bruce, and Alfred hatch a plan to commandeer the prison transport van containing Jennings while wearing ski masks. Halfway across Gotham City, the prison transport van is intercepted by Victor Fries. As Jim Gordon and Alfred Pennyworth end up in a gunfight with Victor Fries, Karen realizes that Victor is after her and throws herself in front of the blast from Victor's freeze gun. Victor then shatters Karen and then uses one of his freeze grenades to cover his escape.
Karen Jennings had black hair and brown eyes. Her most distinguishable feature was her reptilian claw hand, which she received after being experimented on at Pinewood Farms.