https://archive.org/details/CharacterInsightEp204
Sela is the product of a union between Tasha Yar from an alternate timeline and a Romulan General. This Tasha Yar existed thanks to the Enterprise-C going through a temporal rift during a battle with Romulans and creating an alternative timeline where Yar was not dead in the episode Yesterday's Enterprise. When Yar realizes she dies for senseless reasons in the prime timeline, she request leave to join the Enterprise-C crew and go back in time to help them fight the Romulans.
The Enterprise-C is destroyed again, but some of the crew including Yar are taken prisoner. One Romulan General takes a liking to Yar and makes her his consort, and she gives birth to Sela a year later. When Yar tries to escape with Sela a few years later, she is caught and executed. Sela then becomes a military specialist who happens to become a major player in plots to disrupt the Klingon-Federation alliance.
She appears in multiple episodes dealing with these Romulan plots, including The Mind's Eye and the two-part episodes Redemption and Unification. Her plot to supply insurgent Klingons the Duras sisters to overthrow the current Klingon Empire leadership was foiled by Picard's crew, and Picard again foiled her attempt to attack Vulcan with an invasion force in a plot against Spock.
Although the Romulans would eventually turn a clone of Picard against the Federation in a similar movie callback to the Sela character, the captain sees right through his former security chief's daughter and her plots. Just like with the Borg, this character was an interesting antagonist bringing together some of the most important and best episodes of the series, but one which Picard and his crew were especially well-positioned to stop.
The character concept of Sela was created by Denise Crosby, who enjoyed filming Yesterday's Enterprise so much that she wanted a reason to come back on as another recurring character. Little did she know that she would become one of the more memorable villains of the TNG show, and also the only blonde Romulan ever seen on screen in Star Trek. Guess that hair color is the dominant gene in cross-race relations. If you enjoy Sela, she is an often-used villain in the Star Trek books as well.
Sela was played by Denise Crosby, who obviously played Tasha Yar in Season 1 of TNG. Crosby continues to act today, with recent appearances in Castle, Ray Donovan, and The Walking Dead.