>>45408853N is definitely the main antagonist, but I'm not sure if he's really the rival in the usual sense. The way I've always seen it is that Cheren really is the rival character, it's just that BW's plot is the inverse of most Pokemon games, so he doesn't matter as much. Usually the League Challenge is the main plot and the evil team is the B side, but BW is all about becoming the second hero, stopping Team Plasma, and showing N the errors in his thinking. The League Challenge barely matters for the BW protagonist: the credits roll after you destroy Team Plasma and beat N, not after you beat Alder and become the champion.
On the other hand, Cheren's story is about his ideal of strength (contrast with Bianca's story being about her truth of weakness) and begins with his desire to become the strongest, which is what every other Pokemon game is about. Anything that interrupts his quest to prove his supremacy is an annoyance to him; at first he thinks N is just some hippie cook, and being recruited to deal with Team Plasma when they show up is an annoyance. He doesn't take them seriously until Dragonspiral Tower, which the is typical attitude most characters have towards the evil team until the big act 2 reveal when the legendary wakes up. He's also the one that forms a mentor relationship with the champion, similar to Steven and Cynthia in the previous games. Alder helps Cheren realize that there's more to life than winning Pokemon battles and that friendship is what truly matters, which is the moral lesson that virtually every other Pokemon protagonist is supposed to learn, regardless of whether it's the games or the anime or the manga.
tl;dr Cheren is Ash, but nobody cares that he wants to be the very best. Poor Cheren, in any other game he'd be the most important character after the protag but he just can't compete with the king ;_;
