Marcus Caseon

Marcus T. Caseon (or just "Mark") was a doctor who worked at Mars City. One night, a patient came to him complaining of insomnia. Marcus then went to get the right medicine for the patient. When he came back twenty minutes later, the patient had become incredibly hostile and angry and attacked Marcus. Marcus survived the attack. Shortly afterward, he requested to leave Mars. His request is granted. He left shortly before the Demon invasion occurred, on the same ship that delivered Counselor Swann, Jack Campbell, and Doomguy to Mars. Before leaving, he encountered Doomguy. Marcus, still disturbed from his patient's attack, warned Doomguy not to trust anyone on the base, and that strange activities were taking place in Mars City.

Marcus is one of the few characters besides the Protagonist to survive Doom 3, as he (presumably) managed to get off Mars before the demon invasion,

His PDA can be found in the Infirmary the second time the Player visits Mars City. The PDA contains the code for a supply cabinet.