Sgt. Greene was once an active duty soldier, who was in Will's platoon in Southeast Asia, 2009. He stood by Will's side during the Fairchild Republic-USAF contract. Sgt. Greene was stationed at Pratt-Whitney AFO when he was caught in the command building (nicknamed The Hangar)when the apocalypse hit, and stayed.
Sgt. Greene was getting ready at Pratt-Whitney AFO for a training exercize with the other soldiers when the apocalypse was announced, then the Zero Heros took off, leaving them at the base, where they stayed, as they weren't called to duty during the attack.