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)
A version of Marshall created from Leela's memories appears when she is plugged into the simulated world of the computer, Sisyphus. Going by the name Colin Marshall in the Sisyphus's simulation, he and Leela were married with two children, Robbie and Becky, who were used to try to get Leela to accept this false reality and let go of her real memories. When Leela and Jennifer attempted to destroy Crowmarsh, Marshall appeared before her and attempts to convince Leela once more to accept the false reality, but is ultimately unsuccessful as Leela sets fire to the Crowmarsh installation's gas mains, destroying the building and forcing her to wake up. (AUDIO: The Crowmarsh Experiment)