K. Miller and M. Marak are minor UAC Marine characters in Doom 3. They are seen in the Mars City Common Area. The two are talking about the people who are coming and going. When you first meet them, they are talking about a member of Bravo Team, Corporal Allen, who was recently shipped back to Earth due to psychological problems. They wonder who's going to replace him and how long he'll last.
It is likely that during the demon invasion, they set up a defense in the cafeteria but were either killed or turned into zombies.