I mean, he's filling the role of a rival as any rival can get. The player battles him periodically in the games, he is able to be rebattled infinitely at the end of Black and White 2, and he opposes the viewpoints of the player like most rivals in the series. If Wally is considered a rival, then shouldn't N be as well? If you ask me, N fills the role of a rival more than Wally does. In the manga, N is even given a starter Pokemon, so he is given the rival treatment even more there. Just food for thought.