Terry Marsh is an longtime friend of Dr. Cal Lightman who he has a rocky and distant relationship.
It was Terry's family took Lightman in after his mother committed suicide. One time when he and Terry were on the run from the police as kids, Terry acted as a decoy for Lightman, saying to him, "You're supposed to use that brain of yours for something," and then criticizing himself for his way of life. Terry ended up serving three years in prison.
Portrayed by Lennie James.