Kronos is the second-to-last boss that the player must defeat In Doom RPG. He is the mastermind behind the invasion of the base, analogous to Dr. Betruger in Doom 3, and Olivia Pierce in the later Doom (2016).
Kronos was a UAC scientist working on teleportation research. He supposedly traveled 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 transformed 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.
The name "Kronos" is possibly derived from the Greek god of time Chronos, although a titan of similar name (Cronus, the 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.