Kronos was a UAC scientist working on teleportation research. He supposedly travelled to a dimension of pure evil where he combined human technology with demon flesh to create the Cyberdemon. In Reactor, the last level of Doom RPG, it is revealed that Kronos has been changed into a demon.
After the UAC discovered Kronos' research, he was fired and his research buried, but he returned in the guise of Dr. Guerard. Dr. Guerard is encountered early in the game in Sector 1, where he helps the player. However, he sometimes behaves in a suspicious manner, opening doors in apparently impossible ways, for example. Some of the scientists express their suspicions about Guerard, some believing that he is "working for Kronos".
Guerard's true intentions are finally made clear in Sector 6, when he leads the player and Dr. Jensen into a trap. Guerard is encountered again at the entrance to the reactor, where he transforms into Kronos. The player must defeat him to reach the portal and shut it down.
Behind the scenes
Kronos bears some physical similarties to the Mother Demon from Doom 64, but in a more skeletal form. Also the appearance shares some similarities to some of the forms of the Goblin/Abaddon pilot concept that appears in Doom 2016.
The name "Kronos" is possibly derived from the Greek time god Chronos, although a titan of the same name and father of Zeus also exists in Greek mythology. Given the number of geek culture references contained in Doom RPG, it is also possible that the name is a reference to the Klingon homeworld in Star Trek.