Foster was a member of the Army. He was host to two separate Travelers: an unnamed Traveler who was a Faction loyalist; and Traveler 0017, a programmer.
0017 Future History
Traveler 0017 was a programmer responsible for maintaining the Director. He was one of four candidates the Director suspected of being loyal to the Faction, which proved untrue in his case.
Little is known about Foster's life prior to being overwritten. He was a member of the United States Armed Forces, which likely lead to his selection as a host.
Foster was overwritten at some point by a Faction Traveler whose consciousness had been stored in the quantum frame. Along with Jones and Pike, he was captured by the core team and taken to a church, where the trio were overwritten once more.
Pike introduced the three newly arrived Travelers as judges of a tribunal to prosecute Grace Day. Foster questioned the first witness, Grant MacLaren, and interjected a few times during further testimony. He was present when the real traitor, Jones, was apprehended.