The Monitor was an Imperial EF76 Nebulon-B escort frigate that lead an attempted repair operation when Garreth Holtz's flagship, Corrupter, was damaged.
The Monitor provided escort for a convoy of freighters, which were en route to the Mev system to rendezvous with the Corrupter. However, the Monitors presence proved to be insufficient when a Rebel strike force intercepted and destroyed most of the convoy at a transit point outside the system. Monitor herself and several surviving ships made it to the Mev system where she attempted to transfer a team of hyperdrive technicians to the Corrupter. Once again, the Monitor was unable to stop the Rebels as they finished off the convoy and proceeded to destroy the Lambda-class shuttles carrying the technicians. She was destroyed in this battle by Ace Azzameen.
The complete loss of the convoy and the technicians crippled the already damaged Corrupter and prevented her from stopping the Defiance Task Force from escaping the Anoat sector.
It is possible for the player to destroy the Monitor in the Battle of Bettel. Since the ship doesn't appear anywhere else in the game and no other source, it is likely that it was destroyed.