Pvt. Bret Grimes was a part of the Army since 2003. He was with Sgt. Harris before they joined Will's unit in 2009, in Thailand. He was an Infantry Sniper, and could down a man from 1000 yards.
Grimes was at the base early in the morning when the other soldiers arrived, panicked about something. Grimes couldn't understand them, so he continued to do his work, until Sgt. Greene explained it.