The Marine (or Doom Marine) refers to two individuals in the Doom series. In Wolfenstein RPG its prophesied that William Joseph B.J. Blazkowicz's decedents are fated to fight the Harbinger again. Perhaps one or the other fulfilled the prophecy that B.J.'s ancestor would ultimately kill the Harbinger of Doom.
The first is B.J. Blazkowicz as referred in a computer terminal in Doom RPG (though he is usually left unnamed and just called the Marine, or Doom Marine). He is the protagonist of Doom RPG, Doom, Doom II, and Doom 3 (and is based on the UAC Marines of the Doom series) He appears to be an exact clone or relative of Stan Blazkowicz (both share the same sprite and appearance). He was sent to Mars in 2145 in which he fought a Cyberdemon with a soulcube. Late after returning to mars just before the Mars Incident in 2145 (or early 2146), he fought another cyberdemon created by Dr. Kronos. Following the Cyberdemons defeat it was recorded there were no known survivors of the Mars Incident, though he survived and went back to work on Mars. He would again fight a cyberdemon on Deimos, and again on Earth.
This marine would actually fight the Cyberdemon three times, first on Mars at the Mars City (Doom 3), then at the Mars Outpost (Doom RPG), Doom, and again in Doom II, twice in Final Doom, and a few times in Doom 64.
The second is Stan Blazkowicz (Sarge), who possibly fought a cyberdemon in Hell, shortly before confronting VIOS the digital IO/computer virus version of the Icon of Sin.
Technically, either Stan, or another Marine: Kira Morgan, or the Scientist: Riley O'Conner had the chance to fight the Cyberdemon. It is not confirmed which fought the creature exactly.
Behind the scenesEdit
The website for Doom RPG does state that the character from Doom RPG, Doom, Doom II, and Doom 3 are the same character ("you reprise the role of the Doom Marine made famous in the groundbreaking id Software titles Doom, Doom II, and Doom 3"). Chronologically speaking this must mean that Doom 3 occurs before Doom RPG roughly, then Doom I later (Phobos is is forshadowed in Doom RPG), then Doom II. The website describes it as the Marine's "...return to Mars in a showdown with the legions of Hell!".