Sela is the Romulan/Human hybrid who grew up as a Commander in the Romulan Star Empire in Star Trek: The Next Generation.
Events leading upto her 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.
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.
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. 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 instituted a tachyon detection grid at the Klingon-Romulan border, making it impossible for any cloaked ship to cross the border undetected.
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 Captain Picard and Lieutenant Commander Data, he was able to foil Sela's plan and warn the Federation.
Star Trek Online
in Star Trek Online Sela takes a decidedly negative role and is one of the most prominent antagonists in the entire game and arguably becomes the main villain of the entire Star Trek universe as of the end of the Iconian Wars saga, her thirst for vengeance and power causing incalculable loss of death and the shattering of the galaxy, yet in a twist of fate the Iconian War would unite the galaxy again.
Sela is notable for being the longest running villain in the game and unlike her equally vile henchman, Hakeev, she survives relatively unpunished for her countless war crimes and is considerably hated by players as a result (which may in part be Cryptics intention as Sela's wrath and delusion are central to most of the game's overall storyarc).