Zachary Hale Comstock is the leader of the Founders, his own political party, that rules the floating city of Columbia. He is known as 'the Prophet', because his ability to predict the future. Comstock is a deeply religious man, but he views outsiders as a threat to his mission of a New Eden.
Zachary Comstock was formerly known as Booker DeWitt. But after he took the baptism to wash away his sins at Wounded Knee, Comstock forgot about his past and took his new identity.