Arthur Mitchell, often referred to as the "Trinity Killer," is a fictional character and the main antagonist of the fourth season of the Showtime TV series Dexter.
Mitchell is a serial killer who presents himself as an unassuming church deacon, high school teacher and family man.
FBI agent Frank Lundy (Keith Carradine) dubs him the "Trinity Killer" because of a recurring pattern of three killings based on traumatic events during Arthur's childhood.
He forced her, by threatening her husband and children if she does not comply with him, to fall from high above a carefully selected abandoned building to her death.
It is later discovered before these incidents Mitchell also kidnaps a young boy whom he dresses in cowboy pajamas and convinces to play with a train set in a recreation of his childhood.
[3] Mitchell has a family, which includes his wife Sally (Julia Campbell), and their teenage children, Jonah (Brando Eaton) and Rebecca (Vanessa Marano).
[4] At first, Dexter Morgan (Michael C. Hall) is fascinated by Mitchell's efficient killing methods and apparent ability to balance his familial responsibilities with his secret life as a serial killer.
When Mitchell was ten, he spied on his older sister Vera taking a shower; when she saw him, she was so startled that she slipped and fell through the glass door, slicing her femoral artery and bleeding to death.
Mitchell punishes himself after each murder: after killing the woman in the bathtub, he takes a shower in scalding hot water; after forcing a mother to fall to her death, he provokes a drunken man into beating him up.
[6] FBI agent Frank Lundy comes out of retirement to hunt Trinity down but is shot and killed just as he begins to close in on him.
He uses this organization to travel around the country to cover for his murder spree, and (it is later learned) as a dump site for the first victim in each cycle.
He also witnesses Mitchell's strange, unpredictable personality; the same man who murders people with uncommon brutality starts crying when he and Dexter hit a deer, and is appalled at the idea of putting it out of its misery.
He takes Dexter to his old home and tells him that when he was ten years old he startled his sister while spying on her in the shower; she fell and broke the glass door, slicing her femoral artery and bleeding to death.
During Thanksgiving, Mitchell's son, Jonah, lashes out at his father, destroying his homebuilder's plaques and smashing his sister's urn.
After Mitchell learns from a news report that the boy is alive and that Dexter didn't turn him in, he begins to become suspicious.
[4] Miami Metro detective Joey Quinn suspects Dexter of killing Rita, and he connects the dots as to the identity of Kyle Butler.