(covers information from several alternate timelines)
Commander Sela was an officer in the Romulan military, and an extremely important figure in the Empire's covert attempts to destabilize both the Federation and the Klingon Empire. She was the product of a union between Starfleet officer Tasha Yar's counterpart from an alternate timeline and a Romulan general.
Events leading to birth
In the year 2344, while in the middle of a battle with four Romulan Warbirds, the USS Enterprise NCC-1701-C traveled through a temporal rift to the year 2366. This had the effect of creating an alternate timeline, wherein the Federation was at war with the Klingon Empire, and Tasha Yar had not died at Vagra II.
Emerging from the rift, the Enterprise-C encountered its successor, the USS Enterprise NCC-1701-D. Through Guinan, the Enterprise-D crew learned that, in order for the timeline to be restored, the Enterprise-C must return through the rift, even though doing so would almost certainly mean their deaths. Learning from Guinan that, in the other timeline, she had died a meaningless death, Tasha requested and was granted a transfer to the Enterprise-C, which returned through the rift and resumed the battle with the Romulans. (TNG: "Yesterday's Enterprise")
While the Enterprise-C was destroyed, several crewmembers survived and were taken prisoner by the Romulans, Tasha among them. After being interrogated, all of the survivors were to be executed, but a Romulan general became enamored of Tasha, and offered to spare the lives of the prisoners if she became his consort, a condition to which Tasha agreed. One year later, she gave birth to a daughter, Sela. When Sela was four years old, Tasha took her and attempted to escape, but sensing that her mother was attempting to take her away from her home and her father, Sela cried out, and Tasha was discovered and subsequently executed. (TNG: "Redemption II")
Over the next fifteen years, Sela rose through the ranks of the Romulan military, acheiving the rank of commander. In that capacity, she was a major player in plots to disrupt the alliance between the Federation and the Klingon Empire, one of which involved kidnapping Starfleet officer Geordi La Forge, and brainwashing him to kill Klingon governor Vagh. (TNG: "The Mind's Eye") She was also involved in lending support to the Duras sisters in their attempt to gain control of the Klingon Empire, ferrying them materials and supplies in cloaked Romulan vessels. Sela was foiled, however, when Captain Jean-Luc Picard led Starfleet to the Klingon-Romulan border and instituted a tachyon detection grid, making it impossible for any cloaked ship to cross the border undetected. Data, in command of the USS Sutherland managed to triangulate Sela's position and ordered three photon torpedoes scoring a hit on three cloaked Romulan Warbirds, causing them to reveal themselves. (TNG: "Redemption", "Redemption II")
Sela was encountered again in 2368, when she masterminded a plot to invade Vulcan, exploiting Ambassador Spock's underground reunification movement on Romulus. Using three stolen Vulcan ships carrying a Romulan invasion force, she attempted to use Spock to convince the Federation that a peace envoy was en route to Vulcan. Spock refused, however, and, working together with Picard and Data, he was able to foil Sela's plan and warn the Federation. (TNG: "Unification I", "Unification II")
Sela was devised and portrayed by Denise Crosby. In devising the role, Crosby stated, "the part of Sela sort of came about from me sitting around in my house one day thinking about how much fun it was for me to go back and do "Yesterday's Enterprise", and it was so much fun that I thought what else can I do? I thought it was pretty well established that Lt. Yar and Lt. Castillo on "Yesterday's Enterprise" had something going so perhaps they had a child or Yar was pregnant when she went back into the past to fight her final battle. And I sort of thought it out and it seemed to really make sense, and there were no flaws."
"So my original intention was that Lt. Yar would have a daughter that was raised by Romulans and would grow up to try to actually be a Romulan. So I brought this up and the producers really liked the idea and they sort of toyed with it for a while. A few months went by and I got a call and they said we like your idea but we just can't make sense of that Lt. Yar got pregnant by Castillo. We'll have it so that Yar was captured, they didn't all die in the battle, the ship was captured and she was taken by a Romulan general." (Trek: The Next Generation Crew Book)
In her first appearance in the episode "The Mind's Eye", Crosby only lent her voice to this character although uncredited. Actress Debra Dilley appeared as the photo double standing in the shadows. In "Redemption II", Cameron served as hand double for Crosby's character.
Sela has the distinction of being the first (and only) blonde Romulan seen in Star Trek.
Though Sela states how Tasha, her mother, died when she was four for trying to escape, Picard is completely unconvinced that her story is true, and insists it won't affect his judgment in any way. Sela's story is supported by Guinan, who retains memories from the alternate timeline shown in "Yesterday's Enterprise".
Sela returned to plague Captain Jean-Luc Picard and company several more times in the Star Trek novels, notably in Peter David's books Triangle: Imzadi II, in which she attempted to trick Thomas Riker into releasing a nanovirus that would destroy the entire Klingon population on their homeworld; the plan was thwarted by the intervention of William T. Riker, Worf and Odo. Another notable return was in book five of the Double Helix series Double or Nothing. In the book The Romulan Stratagem, Sela forms an alliance with Data to uncover the truth about a series of disastrous events that cast suspicion on both the Romulans and the Federation. She later appears in Michael Jan Friedman's TNG relaunch novel Death in Winter, as an agent of the Romulan government formed by the newly ascended Praetor Tal'aura, whose other allies include Tomalak. During the Typhon Pact series, Sela has become the head of the Tal Shiar, mounting various operations to try and steal the technology for the Quantum slipstream drive from the Federation. After her actions result in the destruction of Deep Space 9 and the Federation and the Pact nearly being pushed to war in the novel Raise the Dawn, Sela is arrested by the current Praetor- who notes that Sela's desire to ensure Romulan supremacy have actually put them in more danger than if she had done nothing-, Sela committing suicide rather than allowing herself to be handed over to the Federation for trial.
The novelization of the two-part "Unification" episode gives the name of the Romulan general who fathers Sela as "Meldet".
Sela also appeared, with the rank of admiral, in the computer game Star Trek: Armada. Further, in the comic book miniseries Star Trek: The Next Generation - The Killing Shadows by Scott Ciencin and Andrew Currie, Sela resurfaces as an exile from Romulus, serving alongside Picard on the planet Nydaris in common cause against a race of ninja-like assassins called the Bodai Shin.
In Star Trek Online, Sela became Proconsul in 2384, but was exiled from Romulus after losing a power struggle with the Tal Shiar. As a result, she survived the supernova of 2387 that destroyed Romulus and killed many of her political opponents. In 2403, she becomes Praetor, and five years later reforms the Empire into a monarchy, with herself as its Empress. In the game itself, the player encounters Sela in the mission "Cutting the Cord," where the player must fight her personal Scimitar-class warbird. Before the player can destroy it, an Iconian ship arrives from an Iconian gateway and tows her ship to an unknown location. She returned in STO's first expansion pack, Legacy of Romulus, in May 2013.