A Dreadnaught-class heavy cruiser owned and operated by an unknown independent party, Mercury at one point aided the Alliance by exiting hyperspace near to where the CR90 corvette Razor was captured by Avenger. She picked up an escape pod carrying a lone Rebel crewman, who had with him the plans for the second Death Star.
Mercury then took the escape pod to the rendezvous point with the Independence and launched the escape pod again to reach the Cruiser. Avenger though showed up again along with the Executor. The crew of the Independence, however, managed to protect the pod while it was moving to the Cruiser. The Mercury fled the scene as it did not possess the firepower to fend off either the Avenger or the Executor.
Behind the scenes
Mercury plays the role of a "deus ex machina" since she inexplicably shows up to help the Rebels in the darkest hour without any introductions. The nature and affiliation of the Mercury, as well as the reason she arrived to the Rebels' aid, is not taken in consideration in the game scenario.