Brady Hartsfield is the main antagonist in Mr Mercedes. Brady was born to a drunk mother and a dad who died in his early childhood. Not much is known about his childhood than the fact that he was forced by his mother to kill his younger brother, his younger brother being severely retarded. This made his relationship with his mother a very strong and even sexual relationship.
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 icecream 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.