Star Trek: The Next Generation's Counselor Deanna Troi (Marina Sirtis) is best known for her time on the USS Enterprise-D, but she's shown up on a surprising number of other Star Trek projects as well. Debuting in TNG's very first episode "Encounter At Farpoint," Troi was a half-human, half-Betazoid officer who primarily tended to the mental health of the ship's crew. Her Betazoid heritage also granted her empathic abilities, which made her a valuable asset when assessing the intentions of potential friends and foes; those abilities earned her a seat on the ship's bridge.
Troi was a difficult character for TNG's writers to crack, and she tended to either fade into the background or feature in tired romance plots in the show's early seasons. The character would eventually find a groove in later seasons, such as in the TNG season four episode "Family" where she helped Captain Jean-Luc Picard (Patrick Stewart) recover from his encounter with the Borg, and the season six episode "Face Of The Enemy" where Troi was unwittingly enlisted into a covert Romulan mission on behalf of Ambassador Spock (Leonard Nimoy). Deanna appeared in all four TNG films, though largely in a supporting role. Yet despite being primarily associated with The Next Generation, Troi has surfaced in other Star Trek series as well.
5 Troi in Star Trek: Voyager
During events set between Star Trek: Insurrection and Star Trek: Nemesis, Troi featured in three episodes of Star Trek: Voyager, each of them also featuring her friend and former patient on the Enterprise-D, Lieutenant Reginald Barclay (Dwight Schultz). In the episode "Pathfinder," Troi visits Barclay on Earth while he's working on a project to make contact with the USS Voyager in the Delta Quadrant. Initially fearing he's having a relapse of his holo-addiction from his days on the Enterprise, she happily celebrates with Barclay when he manages to contact Captain Kathryn Janeway (Kate Mulgrew) and her crew.
Troi would visit Barclay again in the Voyager episode "Life Line," this time on Jupiter Station. She assisted Barclay in his efforts to enlist Voyager's holographic EMH Doctor (Robert Picardo) to help treat the gravely ill Lewis Zimmerman, the doctor who created the EMH program and based its appearance on his own. Troi would visit Barclay one more time in the episode "Inside Man," helping him retrieve a holographic program intended for the USS Voyager's crew that was stolen by a group of Ferengi - and setting him up on a date with a mutual friend of hers named Maril.
4 Troi in Star Trek: Enterprise's Finale
Troi also has the dubious distinction of featuring in one of the most reviled episodes of Star Trek ever, the Star Trek: Enterprise series finale "These Are The Voyages." Set during the events of the TNG season 7 episode "The Pegasus," Troi is seen counseling a conflicted Commander William Riker (Jonathan Frakes), who is struggling with whether he should tell Captain Picard about the top-secret calamity that occurred on the USS Pegasus years earlier. To help him gain perspective, Troi suggests Riker use the holodeck to view the final adventure of Captain Jonathan Archer (Scott Bakula) on the NX-01 Enterprise, just before the formation of what would eventually become the Federation.
Backlash to the episode was swift and harsh. The Enterprise cast - who have been vocal in their displeasure over the episode - essentially end up as bit players in their own finale, the continuity with "The Pegasus" doesn't quite line up, and Sirtis and Frakes are noticeably a decade older than they were during the original TNG episode. What was meant to be a celebration of the end of an era turned out to be a disappointing way to end Enterprise and the Rick Berman era of Star Trek.
3 Troi in Star Trek: Picard Season 1
Riker and Troi consummated their romance by getting married and departing for the USS Titan, Riker's new command, at the conclusion of Star Trek: Nemesis. The couple would have two children while serving on the Titan, Thad and Kestra (Lulu Wilson). Thad became ill with a rare disease, the cure for which was outlawed when the Federation ban on synthetics went into effect. The Rikers retired from Starfleet and relocated to the planet Nepenthe. Unfortunately, the planet's restorative energies were not enough, and Thad passed away as a teenager.
The Rikers were still on the idyllic planet when they were visited by their old friend Jean-Luc Picard and Soji (Isa Briones), the android offspring of Lieutenant Commander Data (Brent Spiner) in the Star Trek: Picard season 1 episode "Nepenthe." Troi broke out her old counselor skills to help Soji cope with the then-new knowledge that she was a synthetic life form. It's one of Marina Sirtis' best performances in the role, kind and empathetic while still clearly carrying the weight of her son's death.
2 Troi in Star Trek: Lower Decks
Troi made a fairly brief appearance in the Star Trek: Lower Decks season 1 finale "No Small Parts." The episode is set between the events of Nemesis and Picard, with Riker and Troi still aboard the Titan. The USS Cerritos found itself in a dire situation, under attack from a much more powerful Pakled vessel. Just before they were destroyed, the Titan warped into the battle to overwhelm the Pakleds and save the Cerritos. While Riker reminisced with his friends Ensign Beckett Mariner (Tawny Newsome) and Captain Carol Freeman (Dawnn Lewis), Troi quickly diagnosed the Cerritos' arrogant First Officer Commander Jack Ransom (Jerry O'Connell) with massive levels of insecurity.
1 Troi in Star Trek: Picard Season 3
Troi is set to return with her fellow Enterprise-D senior staff members for Star Trek: Picard season 3. Little is known about what role Troi will play in the season, and she's been featured sparingly in the show's promotional material. She's most prominently seen sitting around a conference table with her fellow Enterprise-D crew members, one of the most frequent settings from their days on Star Trek: The Next Generation. And while Star Trek: Picard season 3 is likely the final journey for Jean-Luc Picard, there's no guarantee we'll be seeing the last of Deanna Troi, as she clearly has a welcome habit of popping up in unexpected places.