In Halo, MCPON John-117 is Master Chief, a faceless biochemically and cybernetically-enhanced supersoldier who was raised from birth to be a killing machine. He’s rarely seen without his armor or helmet.
Doom’s protagonist is a futuristic marine alternatively called Doom Marine, Doom Slayer, or Doom Guy. Doom Guy originally had no actual name because, according to John Romero, the protagonist is supposed to be you.
In the Alien series, Ellen Ripley (Sigourney Weaver) is a warrant officer aboard the spaceship Nostromo. When the crew wakes from stasis to find an unknown creature has infiltrated the ship, Ripley steps up to play the unlikely hero.
In Aliens, Private Jeanette Vasquez (Jenette Goldstein) is a smartgun operator who’s sent by the Marine Corps to deal with a space colony’s Xenomorph infestation. She’s tough as nails and doesn’t take crap from anyone, a useful skill when you’re in the same squad as Private Hudson.