The Tenth Walker is a common type of story in the Lord of the Rings fandom. In it, an extra character, usually an OC, joins the Fellowship of the Ring (which canonically numbers nine) and travels with them on their quest. As with any type of fic, Tenth Walker stories can be done well, but a Tenth Walker OC is almost always a Mary Sue—a subspecies of the type of Sue that joins the canon heroes in their work, mission, or quest.