You may be looking for Mavek, a Cardassian.

Mavek was a Tilonian working at the Tilonus Institute for Mental Disorders. He was an illusion used by the Tilonians in their attempt to brainwash Riker into giving away classified information about the Enterprise. He was an orderly who constantly tormented Riker both physically and mentally. (TNG: "Frame of Mind")

Mavek was played by Gary Werntz.