Brady Hartsfield

Brady Hartsfield, also known as Mr. Mercedes and The Mercedes Killer, is the titular antagonist of the Stephen King novel Mr. Mercedes. Prior to the main events of the novel, Brady stole a Mercedes-Benz and used it to murder eight individuals waiting for a job opportunity. And after the case has gone cold, he has sent a note antagonizing retired detective Kermit William "Bill" Hodges in his failure to catch him.

Background
Brady grew up with his mother Deborah Ann and his brother Frankie after his father had been killed in a tragic electrocution. Despite having funds from the insurance to keep them afloat, Deborah ultimately squandered much of the money. Things were made worse as Frankie choked on an apple slice and fell into a coma, eventually waking up brain-damaged. Eventually, unable to further put up with the stress of raising a mentally afflicted child, Deborah kicked Frankie's toy firetruck into the basement and nodded at Brady. Taking this cue, Brady kicked his brother down the stairs, killing him in the process. They covered up the incident by claiming Frankie fell, and remained fairly (and disturbingly) close, although Brady considered killing her many times.

Over the years, Brady tinkered with various gadgets and technology, developing remotes that could alter traffic patterns and capture PKE signals to unlock/relock cars (dubbed "Thing One" and "Thing Two", respectively). He used these to cause havoc and steal from others, getting a thrill out of the misfortune he caused. Eventually, he put these to use when, using one of his day jobs as a computer technician for Discount Electronix, he collected the PKE signal of affluent Olivia Trelawney's Mercedes-Benz and stole it. With said Mercedes-Benz, he drove into the City Center and attacked multiple persons awaiting potential job opportunities, killing eight in the process, including a mother and her baby. He wore a clown mask to obscure his identity and cleared away all DNA evidence with bleach. Brady abandoned the Mercedes with a smiley akin to what he would later use in his letter to Hodges.

Mr. Mercedes
Seeking to antagonize the detective who failed to solve the murder, Brady drops a letter in for Hodges, detailing his confession to the Mercedes killings (albeit with exaggerated and/or falsified details to throw Hodges off), in the hopes that the letter would drive Hodges to commit suicide (as the retired detective had considered). Unfortunately for Brady, Hodges decides to once again investigate the case, prompting Brady to take action. In the letter, Brady provided Hodges with an account at a social network site called Debbie's Blue Umbrella, a private site for picture-less, allegedly stress-free chats. In the game of cat-and-mouse that ensues, Hodges riles up Brady by accusing him of and mocking him for attempting serial confession.