Stefopoulos was a member of the United States Colonial Marine Corps. He was part of a 31 marine platoon under the command of Captain Paget, aboard the combat ship Hasdrubal that landed on the planet Tartarus, kick-starting a massive series of events that led to an all out battle for survival against the Yautja species, Xenomorph organisms and the godlike Engineers.
Stefopoulos was part of Sergeant Roth's squad when landing on Tartaturs. After the squad finds a survivor, Seegson asset-stripper Laurence Goode, Captain Paget orders Roth and his squad to go out on a search party and look for any remaining survivors. Roth splits up the team, Stefopoulos, Rucker, Lee and Bonito to sweep the left.
Later, Stefopoulos attempts to contact Roth and inform him that nothing was found, suddenly the Hive Wars Predator appears behind him. Too slow to react, he is later impaled through the back as the other marines turn in shock.