Doomguy nationality

[Dawido]

There are popular theories that DoomSlayer is grandson of Blazkowicz from Wolfenstein, which means that he's Polish-American, which makes sense since his face in DooM Eternal looks pretty slavic, so i can believe that this theory is real

[TacticalBacon]

…

[SuperFly]

Yep, it's true -- though I believe he's a great-grand-descendant of Blazkowicz.
Commander Keen is also in the same bloodline.

More importantly, though, is that DOOMGuy is supposed to be YOU, the player -- so the face avatar is actually unimportant, in that regard.
He certainly isn't God or the Devil, or any of the other useless ♥♥♥♥ you get in the utterly-embarrassing D:E """lore""".

Sadly, the "monster-slaying nephalim" angle introduced in D16 isn't explained or paid off until the sequel, which takes a dive into dumpster fire after that.

[Bane]

Some American chad

[Smugass Braixen-Chan]

BJ is actually Doomguy's Great GREAT Grandfather

BJ Blazkowicz (Wolfenstein Franchise)
Jessie or Zofia Blazkowicz (Wolfenstein: Young Blood)
Billy Blaze (Commander Keen)
Billy Jr. or Billie Blaze (Commander Keen 2019)
Doomguy/Doomslayer