The Romulan Star Empire is the most insidious enemy of Star Trek's United Federation of Planets, and a small number of Romulans have joined or managed to infiltrate Starfleet. The Romulans were among the first alien adversaries introduced in Star Trek: The Original Series. They gained greater prominence in Star Trek: The Next Generation and Star Trek: Deep Space Nine. A Romulan named Nero (Eric Bana) was the main villain of J.J. Abrams' 2009 Star Trek reboot movie, and the Romulans were also the big bads of Star Trek: Picard season 1.
The Romulans and their secret police, the Tal Shiar, have attempted many dastardly means to subvert and upend Starfleet, with a surprising amount of success. One failed Romulan plot, however, was their attempt to replace Captain Jean-Luc Picard (Patrick Stewart) with his clone, Shinzon (Tom Hardy), in Star Trek: Nemesis. The most recent Romulan scheme was foiled by Admiral Picard in Star Trek: Picard season 1, which also introduced two close allies of Jean-Luc's: ex-Tal Shiar members Laris (Orla Brady) and Zhaban (Jamie McShane). Zhaban was killed off before Star Trek: Picard season 2, and Laris became Jean-Luc's new love interest. Meanwhile, here are the Romulans who made inroads into Starfleet.
4 Elnor
Elnor (Evan Evagora) was hailed by Admiral Picard as the first full-blooded Romulan in Starfleet. Elnor was introduced in Star Trek: Picard season 1 as a young boy on the planet Vashti, where the sect of Romulan warrior nuns called the Qowat Milat relocated when the Romulan sun went supernova. Jean-Luc befriended and mentored the young Elnor, who was accepted by and became a rare male member of the Qowat Milat. The adult Elnor later made Picard his "lost cause" and joined Jean-Luc's motley crew to protect Soji (Isa Briones) and the synthetics of the planet Coppelius from the fanatical Romulan cabal called the Zhat Vash.
Elnor was accepted into Starfleet Academy as a Cadet in between Star Trek: Picard seasons 1 and 2. Cadet Elnor was assigned to the USS Excelsior under Commander Raffi Musiker (Michelle Hurd), but he was later killed after Picard and his friends were brought to an alternate Confederation reality created by Q (John de Lancie). Happily, the dying Q used some of his remaining energies to resurrect Elnor in Star Trek: Picard season 2's finale. Elnor is not part of Star Trek: Picard season 3, but the young Romulan warrior is presumably still serving on the Excelsior.
3 Narissa AKA Lieutenant Rizzo
Colonel Narissa (Peyton List) was a member of the synthetics-hating Zhat Vash sect of the Tal Shiar. Narissa infiltrated Starfleet disguised as a human named Rizzo, and she rose to become a Lieutenant overseen by her handler, Commodore Oh (Tamlyn Tomita). It's unknown how long Narissa posed as Lt. Rizzo, but in Star Trek: Picard season 1's premiere, Narissa led a Romulan assault team that murdered Dahj Asha (Isa Briones), the twin sister of Soji, after she learned she was synthetic and sought Jean-Luc Picard for help.
Narissa, in turn, oversaw her brother Narek (Harry Treadaway), who infiltrated the Borg Cube Artifact. Narek's mission was to glean the location of the synthetics' homeworld, Coppelius, from Soji, but the Romulan became infatuated with the beautiful synth scientist. Narissa eventually tried to kill Soji herself, and she murdered thousands of Borg drones in the Artifact. Seven of Nine (Jeri Ryan) ultimately killed Narissa at the end of Star Trek: Picard season 1.
2 Commodore Oh AKA General Nedar
No Romulan did as much damage to Starfleet and the Federation from within as Commodore Oh. Oh is really General Nedar, the leader of the Romulan Zhat Vash. In Star Trek: Picard season 1, Oh held the powerful high rank of Starfleet Chief of Security, and she presumably infiltrated Starfleet decades prior. It was Oh who covertly orchestrated the Mars Attack of 2385 that killed 90,000 people and was blamed on rogue androids. The Mars Attack resulted in the Federation banning artificial lifeforms for almost 15 years, which held with Oh and the Zhat Vash's own hatred of synthetics.
Commodore Oh finally dropped her guise to lead a Romulan fleet against the synthetics living on the planet Coppelius in Star Trek: Picard season 1's finale. Oh's plan ultimately failed thanks to Jean-Luc Picard and his crew, while her invasion was stopped by Captain Will Riker (Jonathan Frakes), who countered with a Starfleet armada of his own. Star Trek: Picard has not fully accounted for the havoc Commodore Oh caused in Starfleet. Meanwhile, Oh/Nedar is purportedly half-Vulcan and half-Romulan, which helped her rise up the ranks to literally alter the course of the Federation for years.
1 Lt. Saavik
Lt. Saavik was portayed by Kirstie Alley in Star Trek II: The Wrath of Khan, and by Robin Curtis in Star Trek III: The Search for Spock and Star Trek IV: The Voyage Home. The protégé of Captain Spock (Leonard Nimoy), Saavik joined the crew of the USS Enterprise under the command of Admiral James T. Kirk (William Shatner). Although it wasn't explicitly stated in Star Trek II, Saavik was half-Vulcan and half-Romulan, and the latter side of her genetic makeup was evident when she shed tears after Spock's death at the end of the film.
In Star Trek III, Saavik joined Kirk's son, Dr. David Marcus (Merritt Butrick), on the USS Grissom to survey the Genesis Planet, where they found Spock resurrected by the Genesis Device's life-giving energies. After Kirk and the Enterprise crew rescued Spock, Saavik joined them on their voyage to Vulcan to reunite Spock with his katra (his Vulcan soul). Saavik remained on Vulcan when Kirk and his crew returned to Earth at the start of Star Trek IV, but a recent canonical update revealed that Saavik became Captain of the first USS Titan in Star Trek.