Bill Calhoun was a member of the Followers of the Apocalypse.
Background
In 2246, he and Edward Sallow (the future Caesar), were sent east to study tribal dialects. He was instructed to meet with a Mormon missionary named Joshua Graham who was a tribal specialist. They embarked on a journey to the region known formerly as Arizona as part of a nine-person expedition.
At some point in 2247, Graham, Calhoun and Sallow were captured by the Blackfoot tribe for ransom. At the time the tribe was at war with seven other tribes, a war they were inevitably going to lose. Unwilling to be destroyed along with them, and against the wishes of his companions, Sallow chose to use his knowledge to train the Blackfoot tribe in the art of warfare after witnessing their lack of knowledge first hand. He quickly impressed them enough to the point where he was made their leader, conquering other tribes.
Surprisingly, Joshua Graham decided to join Sallow, now known as Caesar, as his right-hand man, becoming Caesar's Malpais Legate. While Calhoun was sent back to the Followers to inform them of what he was doing, the other six members of the expedition were killed on the self-proclaimed Emperor's orders.
Appearances
Bill Calhoun is only mentioned in Fallout: New Vegas and his full name is given in the add-on Honest Hearts.