Jubal is the mentor to just about everybody. Sometimes it feels like every other chapter in the book is Jubal sitting down with someone and teaching them to be more open-minded and reconsider his or her preconceived notions. Snore. Some critics have argued that Jubal is actually the protagonist of Stranger, since he receives more screen time than Mike. While this reading may prove true—it's definitely interesting—we at Shmoop are going to place him in the Mentor role (for the moment at least).
Foster plays a small role as the angelic mentor to the deceased Digby. He is, more-or-less, the afterlife's Jubal. Whether or not he becomes Mike's mentor, or Mike becomes his, is left open at the end of the novel.