What is the name of the Trinity Killer?

The Trinity Killer’s real name is Arthur Mitchell. He is the main antagonist of Dexter’s fourth season and is played by John Lithgow.

Arthur appears to be a respectable family man: he works as a high-school teacher, serves as a church deacon, and leads the Four Walls One Heart charity. Behind that public identity, he is a prolific serial killer whose ritual killings follow a recurring pattern involving a bathtub death, a fatal fall, and a hammer attack. FBI agent Frank Lundy gives him the nickname “Trinity” after identifying the apparent three-part pattern.

Dexter initially studies Arthur because he seems to balance family life with serial murder. Using the alias Kyle Butler, Dexter befriends him and observes his household. He eventually learns that Arthur’s killings are tied to childhood trauma involving the deaths of his sister, mother, and father.

Viewers sometimes confuse Arthur Mitchell with other season villains such as Miguel Prado or Jordan Chase. Arthur is specifically the Trinity Killer, and his murder of Rita becomes one of the most consequential events in the series.