He was portrayed by Bruce French.
A full Betazoid, Genestra was a skilled telepath. In the 2360s, he served as an aide to Admiral Norah Satie until she retired from Starfleet. In 2367 after an explosion on the USS Enterprise-D Satie was asked to return to service to help investigate the explosion, she requested that her former staff be reassigned to her for the duration of the mission. Sabin accompanied Admiral Satie to the Enterprise.
Once on board the Enterprise working with Worf Genestra quickly determined that the Klingon officer J'Dan was a spy for the Romulans. Genestra told Worf that he had some misgivings about Worf due to his father being condemned as a traitor by the High Council, but that he was convinced that Worf was not a security threat.
J'Dan admitted that he was a spy, but he insisted he had not damaged the warp core. Concerned over a possible conspiracy on board, Captain Picard and Admiral Satie decided to expand the investigation and began interviewing crew members who had contact with J'Dan. When crewman Simon Tarses was interviewed Genestra sensed deception in the young man.
At about the same time the radiation levels in the warp core came down to the point that the crew was able to gain access to the warp core. Commanders Data and LaForge soon discovered that the explosion was not a deliberate act, that when the core hatch was replaced following the battle with the Borg it was replaced with one that was defective. The hatch happened to fail just as J'Dan was carrying out his mission for the Romulans.
Even though the hatch explosion was an accident, both Satie and Genestra wanted to press on with the investigations. Genestra uncovered what had Tarses so worried - he had a Romulan grandfather instead of a Vulcan grandfather when applying for admission to Starfleet. However instead of being truthful Tarses had lied and stated his grandfather was Vulcan. Picard met with the young man and came away convinced that Tarses was not a traitor, just a young man who made a serious mistake. When Picard tried to stop the investigations into Tarses Satie told him that the hearings would be expanded and observed by Admiral Thomas Henry of Starfleet Security.
In short order Picard was brought before Satie and her staff. There Satie proceeded to attack Picard relentlessly. When Worf tried to defend his Captain to Satie and Genestra the Betazoid then told the Captain that he showed questionable judgement in keeping a security chief whose father was a Romulan spy. Worf was about to physically demonstrate to Genestra what he thought of Genestra's attacks on his father but was held back by a word from Picard.
Picard proceeded to quote the words of Admiral Satie's father on freedom and the rights of individuals. This prompted Satie to launch into a furious rant about protecting the Federation and that she had brought down bigger men than Picard. Totally disgusted Admiral Henry got up and left the hearing, leading Genestra to call a recess. Henry called an end to the hearings. Genestra and Satie soon departed the Enterprise.
Genestra returned to Starfleet and continued serving as an aide to flag personnel. He served a total of seven different Admirals by 2379, and received nothing but praise from each of them. Admiral Tujiro Nakamura asked him to be part of an inspection team that was examining the Enterprise-E in detail. Genestra was assigned to investigate security and personnel on the Enterprise. Over the next week he interviewed a number of Enterprise personnel, and purposely angered Lt. Commander Vale to judge her reaction. Afterwards he gave the Enterprise a passing grade for his part of the inspection.