The Killer In the Backseat is the titular main antagonist of the urban legend "Killer In the Backseat".
The legend involves a woman who is driving and being followed by a strange car or truck. The mysterious pursuer flashes his high beams, tailgates her, and sometimes even rams her vehicle. When she finally makes it home, she realizes that the driver was trying to warn her that there was a man hiding in her back seat.
Each time the man sat up to attack her, the driver behind had used his high beams to scare the killer.
In one version, the woman stops for gas, and the attendant asks her to come inside to sort out a problem with her credit card. After she gets out of the car and starts to follow the attendant, he tells her that there's someone in the backseat and that they are going to the cashier's booth, locking all of the doors, and calling the police.
The story is often told with a moral. The attendant is often a lumberjack, a trucker, or a scary-looking man; someone the driver mistrusts without reason. She assumes it is the attendant who wants to do her harm, when in reality it is he who saves her life.