Who is Jonathan Nye from Mount Dragon: A Novel and what is their importance?

Jonathan appears in the hallucination that Nye sees when he is overcome by PurBlood. He was Nye's older brother, who drown while swimming with Nye as a resuly of being pulled under by a wave. In Nye's hallucination, he reveals himself to Nye and becomes his ever more visible companion as they track Carson and de Vaca, and then afterward as they head out together to retrieve Mondragon's gold.