Commander Sela, played by Denise Crosby
Alternate timelines are always so interesting aren't they? Although she died in “Skin of evil”, in another future opened up during “Yesterday's Enterprise”, Tasha Yar returned with the Enterprise-C in order to balance out the timelines. Seems she was not then killed, but captured by Romulans, one of whom fell in love with and married her. The union of that marriage was Sela, who looks just like her mother (well, she
is played by Denise Crosby!) other than the Romulan ears. She is in fact almost a female Spock, half-Romulan and half-human. She tells Picard that her mother died when trying to escape with her, and due to the warning shouted out by Sela was recaptured and later executed. Sela does not evince any emotion at the death of her mother, and it is clear that she thinks of her as a traitor to what she now sees as her own people. She is first encountered in TNG when she captures Geordi LaForge and brainwashes him, turning him into a “Manchurian Candidate”-style assassin. Later she helps Lursa and B'Etor to stage a coup and seize control of the Klingon Empire, by gunrunning but the plan is foiled by Picard and particularly Data, who detects the cloaked ships trying to cross the Klingon/Romulan border.
Picard and his crew next encounter her when, in the process of tracking down the said Ambassador Spock on Romulus, they are both taken prisoner by her as she puts into action her plan to sneak Romulan shock troops into Vulcan as a prelude to invasion and conquest. She is of course defeated, but this does not stop her trying. This is the last we hear of her in the series, though apparently one of the novels has her committing suicide rather than be handed over to the Federation when she engineers the destruction of Deep Space 9. The little hussy!