A Romulan admiral who defected to the Federation to warn of a planned attack, only to find he had been fed false information.
Alidar Jarok fled into Federation space in The Defector, at first claiming to be a minor logistics officer named Setal. He warned that the Romulans were building a base in the Neutral Zone on Nelvana III, and he eventually revealed that he was in fact an admiral.
When the Enterprise-D reached Nelvana III, there was no base, and Tomalak arrived to capture the ship. Jarok's superiors had tested his loyalty by allowing him to see false plans. Picard's secret arrangement with Klingon allies forced Tomalak back, but Jarok, unable to return home, took his own life, leaving a letter for his family.