A Romulan commander and the daughter of Tasha Yar from an alternate timeline.
Sela is a Romulan commander in The Next Generation, played by Denise Crosby. She is the daughter of an alternate-timeline Tasha Yar: in Yesterday's Enterprise, that Tasha goes back in time aboard the Enterprise-C and is captured by the Romulans. She bears a daughter to a Romulan general and is executed when Sela is still a child.
Sela first appears in silhouette at the end of The Mind's Eye and makes her full debut in the two-part Redemption, where she secretly supplies the House of Duras during the Klingon civil war. Her plan collapses when Picard's fleet uses a tachyon detection grid to expose the cloaked Romulan supply ships.
She returns in the two-part Unification, using Spock's reunification movement as cover for a planned Romulan invasion of Vulcan, which Picard and Spock defeat. Sela is a cold, ambitious adversary whose existence ties together the franchise's themes of alternate histories and lost friends.