Dr. Kelvin is a scientist working for the UAC on Mars. His assistant is Jenkins.
Dr. Kelvin is first found in Sector 4 (Doom RPG), where he is struggling to open a locked door, finally deciding to blast it open by asking the player to fire at barrels that have been placed by Jenkins. This has no effect and only results in the computer controlling the door mocking the player with the message "HAHAHA! You thought you could blow me up?!?!", until the mysterious appearance of Dr. Guerard, who gets the door open.
Dr. Kelvin assists the player several times throughout the game; for instance when the player needs to be teleported to the other side of a lab in Sector 7 (Doom RPG). He is last encountered 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 "One of those things got inside of me. I can feel it", then transforms into a monster (you can avoid it, just don't talk to Kelvin after you opened the door). The monster has to be eliminated by the player before exiting the level.