Dr. Kelvin

Dr. Kelvin is a scientist working for the UAC on Mars. His assistant is Jenkins.

He's first found in Sector 4 (Doom RPG), where he's struggling to get open a locked door, and decides finally to blast it open by asking the player to fire at barrels that have been placed by Jenkins, this with no effect and resulting in the computer controlling the door mocking at the player with the message "HAHAHA! You thought you could blow me up?!?!", until the misterious appearance of Dr. Guerard, who gets the door open.

He assists the player several times throughout his journey, like when the player needs to be teleported to the other side of a lab in Sector 7 (Doom RPG), and he's last seen at the destroyed Junction (Doom RPG) after the second attack, when guarding the door that leads to the Reactor (Doom RPG) trying to prevent more monsters from getting inside. At the moment the player gets the door open, Dr. Kelvin says that "one of those things got inside of me. I can feel it", then transforms into a monster, and has to be killed by the player, before exiting the level.