Kronos

In Doom RPG, Kronos is the second-to-last boss that the player must defeat. He is the mastermind behind the invasion of the base, somewhat analagous to Dr. Betruger in Doom 3.

Spoiler Warning: Plot details follow.

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.