Marshall was a member of the Rocket Men.
He checked the Fourth Doctor's cell so that he couldn't escape. He didn't believe that Leela was a bounty hunter. He was stolen as a child, made to work for the Rocket Men and wanted to use the Doctor as a way out. (AUDIO: Requiem for the Rocket Men)
He contacted the Doctor when Leela went missing, and told him to meet him at the Death Match. Vargrave placed him inside the game. He found Leela in the game. He had his legs broken by the Red Knight. He died to destroy the Death Match, saving the Doctor and Leela. (AUDIO: Death Match)