Why had Johannes been such a valuable friend to David?

Johannes, whose last name and nationality are never revealed by the narrative, is a fellow inmate in the internment camp where David spends his childhood. Educated and articulate, wise and a mentor, he instructs David in the ways and philosophies of personal integrity, morals, and self-worth. He offers guidance to David throughout his journey, both in terms of what David remembers of him and also, early in the journey, as a presence that seems almost ghostly, or spiritual.