The Romulan commander who defects to the Federation in “The Defector” is Admiral Alidar Jarok. He initially conceals his identity by posing as Sub-Lieutenant Setal, a logistics clerk seeking asylum aboard the Enterprise-D.
Jarok claims that the Romulans are preparing an invasion from a secret installation on Nelvana III. Captain Picard is skeptical, but Jarok eventually reveals that he is a senior officer who previously led attacks against Federation outposts. He says he defected because he believes the Romulan government is preparing a disastrous war and wants to protect his family from its consequences.
The supposed installation turns out to be a deception. The Romulans use Jarok’s intelligence to lure the Enterprise into the Neutral Zone, where warbirds attack. Picard counters by calling in Klingon support, preventing the trap from becoming a defeat.
The episode ends tragically: after discovering that he was manipulated, Jarok takes his own life. His courage and remorse make him one of the more memorable Romulan figures in The Next Generation.