Bill is a cowboy in the spirit movie and appears to be the leader  of the group that caught spirit .He is first seen sleeping ,he is also the one to give the order to chase the buckskin stallion spirit, in the chase he is throw from his saddle, when spirit is first roped he gets his back leg, he appears to be leading the other to the cavalry and is the one who takes the money from Sgt Adams

According to the game "Spirit: Stallion of the Cimarron, Forever Free"  Bill and the Wranglers have a place called "Snakefinger's Pit", an unpleasent canyon where they keep their caught horses.