Mark B. Ryan (Identification Number: 9368-41) was a Mars-Sec security officer assigned to the Mars City Underground on Mars. He guarded the airlock connecting the Underground and the abandoned communications facility. Mark wore a standard Mars-Sec uniform and full helmet, and had a standard PDA.
When the player meets Mark Ryan in Doom 3, he is reading his PDA at the airlock entrance. Having already been told by T. Brooks that the player was coming ahead of time, Mark directs the player towards the communications facility and assuages his fears about having to walk across the surface of Mars by reminding him that his suit carries plenty of oxygen.
During the demon invasion, when the player enters the airlock to the Underground, it malfunctions and only opens enough for the player to see a dead Mark getting dragged away by a zombie. The airlock then opens enough for the player to exit the airlock and kill the zombie.
Got a new guy on the way (11-15-2145)
We had a new marine just come through the security checkpoint. He's gonna check out the old comm building for that missing scientist. Can you make sure he doesn't get lost... last thing we need is another missing person.