Brady Hartsfield is the main antagonist in Mr. Mercedes. Brady was born to an alcoholic mother and a father who died in his early childhood. Not much is known about his childhood , other than the fact that he was forced by his mother to kill his younger brother, who was severely retarded. The result is a strongly sexual relationship with his mother.

Brady Hartsfeild is first seen committing the famous Mercedes killing by running over and killing 8 people at a job fair, giving him the name of the Mercedes Killer. Here it is shown that he is a very Jon Doe-esque murderer, leaving many clues and notes to the cops, including a smiley face on the dashboard of the Mercedes, his sigil. He worked as a electronics salesman and an ice cream truck driver, the latter to spy on Bill Hodges, who was the detective on his murder case. Bill Hodges had since retired, but Brady kept sending him anonymous notes warning of his next strike. Brady is seen to be an emotionally unstable and possibly mentally ill young man.

Brady makes a brief appreciate in Finders Keepers when Bill Hodges visits Brady's hospital room after he was seriously injured by Holly Gibney. It is shown that Brady has gained the ability to move objects with his mind.

In End Of Watch it is told that Brady gained his abilities from combination of medical drugs and brain damage from Holly Gibney. We learn that Brady also has the ability to possess other humans who are under the influence of a Zappit game console. This leads to Brady using his abilities to cause the suicides of people who he had previously attempted but failed to kill. Brady plays another cat-and-mouse game with Bill Hodges but fails and is killed by Jerome Robinson.