Marlena Moreau (mirror)

For canonical information see the link below.

By most accounts she underwent a change of loyalties after the return of her captain's counterpart to the Federation (Dark Mirror), and in The Mirror Universe Saga she served in David Marcus' underground. It may have been she who convinced the Drs. Marcus to question the Empire, because in that timeline Spock did not come around to that way of thinking until much later.

However, the Mirror Universe Trilogy had her pretty much suffer the same fate as Spock did in Dark Mirror.

For canon information see Marlena Moreau (mirror)