Daniel Truman (portrayed by Brian White) was the person responsible for security in the control room where Hadley and Sitterson controlled the cabin in the woods. He was the new person and so asked questions pertaining to the ritual, such as the reason for the choice and the fact that the Virgin's death was optional. Noticeably, Truman took the proceedings more seriously than the others, refusing to enter the betting pool and thinking the engineered sex scene with Jules was crass. (Although he is shown watching the scene with intrigue, showing that even he's not completely above it.)
During the System Purge, Truman fought off several monsters before being attacked by the Scarecrow Folk, sacrificing himself to save the others by blowing him and them up with a grenade.
His name is probably a reference to The Truman Show, a 1998 movie starring Jim Carrey as a man who figures out his entire life is a television show.