The Outcast by Simon Hawke

Cast out to die in the Athasian desert, a half-breed child is rescued by a wandering druid and brought to a hidden temple in the Ringing Mountains. Raised by the mystical villichi, Sorak learns the Way of the Druid and the Path of the Preserver, but his ordeal in the desert has fragmented his personality. He has become a "tribe of one," possessing multiple identities within one body, each with its own voice and hidden attributes. As he struggles to resolve his inner conflict, Sorak embarks on a quest to find his origins and seek out a powerful preserver known only as the Sage, who has been marked for death.