Phobos is a likely African-American (considering the suggested United States-background of the Marines), Marine who appeared in Quake III Arena, and is meant to be tied to the events of the original Doom.
According to information from the Quake III Arena manual, Phobos was an experienced veteran who went to Phobos with the rest of his team (including the later Doom Marine) He was a courageous, determined Officer that was also a "Gentleman", and perished there with most of his team.
Inaccurately, the manual states that Phobos was betrayed by his superiors who "rather watch him die, then studying the reports" which makes no sense (in the original game), especially considering that there were no survivors on Phobos (at least in the original DooM game) and they were the only combat team on the moonbase.
Phobos is only canon in the Quake III Arena universe, so whether he appeared in the original barebones Doom story is entirely up-to the player.
Behind the scenes
- Phobos is probably a characterization of the 4th player's reddish-orange color in the original DOOM's co-op mode.
- A figure of Phobos was released along with the "Doomguy" from Doom as a Doom-related set by Symbiotes.
- A Figurine can be found in the 2016 DOOM reboot, called Phobosguy, sharing the same iconic color scheme.