THX 1138 (Robert Duvall): He is a factory worker who is unknowingly taken off his mood drugs by his roommate. Once off the drugs, he discovers strong emotions such as passion, which leads him to make a series of decisions that go against what society dictates.
SEN 5241 (Donald Pleasence): An older member of the drab society who sees THX 1138 as a kindred spirit. He joins THX 1138 in an attempted escape.
LUH 3417 (Maggie McOmie): THX 1138's roommate; she takes herself and THX 1138 off of their medication. A physical relationship with THX 1138 develops after strong emotions appear. The result has dire consequences for her.
SRT, the hologram (Don Pedro Colley): He joins THX 1138 and SEN 5241 on their quest to escape from the suffocating society.