The doctor treating Riker in a mental institution that Riker comes to believe is real, during a covert mission to Tilonus IV.
Syrus is the doctor who treats Riker in the asylum where he finds himself after a performance of the play Frame of Mind. He insists that Riker is a patient who killed a man and that his memories of the Enterprise are delusions.
In Frame of Mind, Riker's mind keeps shifting between the asylum and his starship, and Syrus works to convince him that the asylum is reality. Riker finally discovers that he was captured during an undercover mission on Tilonus IV and that his captors are using a neural extraction device to probe his mind.