B.J. Blazkowicz

B.J. Blazkowicz is a Doom Marine in Doom RPG, possibly the name of the Marine, the player character in the game. You find the name mentioned a log addressed to B.J. Blazkowicz discussing a hidden secret room containing a shotgun for the Marine to find.

Doom Marine
The website for Doom RPG does state that the player in Doom RPG is the same character from Doom, Doom II, and Doom 3. "You reprise the role of the Doom Marine made famous in the groundbreaking id Software titles Doom, Doom II, and Doom 3." Although its unclear which order they fit if part of a single timeline. Perhaps Doom 3 is rougly the prequel, followed by Doom RPG, and then Doom I, and then Doom II.

He is the descendent of William B.J. Blazkowicz, the World War II veteran who stars in the Wolfenstein series, and William Blazcowicz "Billy Blaze" II, the protagonist of Commander Keen. His cousin or relative is Stan Blazkowicz of Doom II RPG.

Behind the scenes
According to the Doom RPG website "you reprise the role of the Doom Marine made famous in the groundbreaking id Software titles Doom, Doom II, and Doom 3.", suggesting that the Doom RPG stars Doom's protagonist.