Marsen was a male lieutenant in service to the Galactic Empire in 3 ABY. He was present during the Battle of Hoth in which he was acting as a lieutenant with the Imperial ground battle group. When Lord Starkiller killed Captain Keenah, Marsen was assigned to Starkiller as his new liaison for the rest of the battle. During Starkiller's mission, Marsen provided him with technical details of Rebel technology.
In 3 ABY, Marsen served as a lieutenant in the Galactic Empire. When the Galactic Empire acknowledged that the planet Hoth had a Rebel presence during that year, an Imperial armada was sent to the planet to wipe out the Rebel presence. The Empire dispatched Starkiller with the Imperial force, knowing that the young Rebel Jedi who destroyed the Death Star, Luke Skywalker was present with the Alliance. During the the ensuing battle, Marsen directed the Imperial's ground group assault against Echo Base. During the battle, Starkiller killed his personal liaison, Captain Keenah, and Marsen was assigned as Keenah's replacement.
At some point during the mission, Starkiller encountered a shield generator, which prevented him from penetrating the main base. Marsen contacted Starkiller, describing how to disable the shields.
After Starkiller penetrated the shield barriers, he arrived in the center of Echo Base, where he quickly dispatched the Rebels present there. Marsen then informed Starkiller that their forces had destroyed the shield generator that protected the base and that their troops would be arriving in moments.
As Starkiller continued dispatching Rebels, Marsen informed him that their commander, Luke Skywalker, was nearby, and relayed to Starkiller that the Emperor wanted Skywalker alive if possible. After Starkiller reached another Rebel command center, 501st troops arrived to assist him; Marsen directed the Sith Lord to the principal Rebel hangar, where Starkiller fought Skywalker, eventually turning him to the dark side.
Marsen, a lieutenant serving in the Imperial Military, was assigned to the Empire's ground forces during the Battle of Hoth. He was designated as Starkiller's new personal liaison following the hapless Keenah's execution, and quickly proved himself a bit more competent, demonstrating his knowledge of mechanics and shield generators while assisting Starkiller.
Lietuenant Marsen made his first appearance in the non-canon Hoth scenario in Star Wars: The Force Unleashed: Ultimate Sith Edition.