The Rebel Marine is a marine class unit for the Rebel Alliance, it only available on space combat modes. They can pack some serious ammo, but they are not quite as popular as the pilot class, because they cannot fix the ships that they are piloting. They are good for taking out enemy units, and they are pretty good at taking out the critical points inside the enemy command ship, but they are generally overshadowed by the pilot class in games. In addition, they are only playable during assault mode. They look like Rebel Fleet Troopers seen on Tantive IV on a level of The Rise of the Empire (Equivalent- Imperial Marine, Droid Marine, Clone Marine)
The Rebel Marine is actually worse at destroying enemy systems than the Rebel Pilot. The Pilot's time bombs are considerably more efficient than the Marine's rockets. However, the Marine is still useful as their rifle is far better for extended firefights on board a capital ship than the Pilot's pistol.