Sela is a half-Human, half-Romulan Commander in the navy of the Romulan Star Empire. She is the daughter of Tasha Yar and an unnamed Romulan commander who participated in the Battle of Narendra III. She was born in 2345, sometime after Tasha Yar was captured in the aftermath of the destruction of the Enterprise-C at Narendra III.
Klingon Civil War
Sela commanded a fleet of Romulan Warbirds that secretly supplied the Duras faction during the Klingon Civil War (TNG "Redemption"). Her efforts were initially successful, but the Federation deployed a fleet of starships to blockade the Klingon-Romulan border, using a tachyon net to detect cloaked ships attempting to run the blockade. A Romulan attempt to disrupt the net and rush their fleet through failed, revealing that the Duras faction was receiving Romulan assistance and destroying the family's reputation.
Sela participated in a plan to land Romulan troops on Vulcan, creating political chaos in the Federation (TNG "Unification"). The plan involved luring Ambassador Spock to Romulus to discuss the possibility of opening formal relations between the Vulcans and Romulans. After some discussions, Spock was captured along with Jean-Luc Picard and Data, who had followed him to Romulus to determine why he had gone there. Sela revealed that Romulan troops would be moved to Vulcan on reconstructed Vulcan transports, and she demanded that Spock send a message announcing that the ships were on a peaceful mission to establish an embassy. When Spock refused, she revealed that during his earlier discussions, the Romulans had recorded his voice and appearance, which they would use to holographically generate the message without his help. Sela blundered by leaving Spock, Picard, and Data alone with an unguarded computer console, which they used to corrupt Sela's message and arrange their escape.
Sela is an impetuous commander who develops excessively complicated plans and then overlooks important details in their execution. She is proud of her Romulan ancestry and embarrassed by her Human parentage, resulting in a grudge against the Federation. Her intelligence does not seem to be exceptional, nor is there evidence of impressive military victories in which she participated, so it's unclear how she obtained high rank at her young age.
- Sela on Memory Alpha.