Doctor Syrus was supposedly a psychiatrist on the planet Tilonus IV.
He worked in Ward 47 of the Tilonus Institute for Mental Disorders. In truth, Syrus was a character in a delusion created by a neurosomatic technique used by Suna, who was trying to extract tactical information from Commander William T. Riker. (TNG: "Frame of Mind")
Syrus was played by David Selburg.
He was called Osyrus in the first draft script.