Alexandra was a terran who served the rebel/terrorist Cliff Nadaner, who was an opponent to the Terran Dominion. A few years after the Brood War, she and Nadaner's band were hiding out on Tyrador VIII. Their location had been discovered however, and Alexandra was killed by a ghost sent to neutralize them.
References
DeCandido, Keith R. A. (November 28, 2006). StarCraft: Ghost: Nova. Simon & Schuster (Pocket Star). ISBN 0-7434-7134-2.