Partner Rival and Pokemon: (For this one it'd be good if you included a list of each rival's Pokémon, just so it was easier to find out which one to choose) Gary and Blastoise.
Reason for joining: Apart from the main characters, the rivals are usually the ones we get to know the most about. Throughout the episodes that they're in, we get to know their personality, in which a downer is that many of the personalities are alike, though there are some who varies from others. We're also usually provided with a backstory of some kind, which also gives us a way to relate more to the rivals, than the minor characters that usually only appear in one episode.
Among the rivals, who is your favorite and why?
This one is kind of a tie between Gary and Richie. Gary is the first and the original rival, both in the games and in the animé, and I usually go with liking the first ones I got to know the best, and since Blue was my first game, and Gary was based off of the in-game rival, I'm bound to like him. Other than that he has a great personality, he is very confident in himself, though he still aknowledge it when he loses.
Even though he at first glance appears to be an unreasonable little brat, he actually is pretty mature, and he is also pretty intelligent, which becoming a professor has shown.
Richie is also one of the original rivals, and to be honest, he isn't that much of a rival, mostly just a friend and a temporary companion. He's one of Ash's best friends, and is a really nice person overall, and has the best of the personalities imo. His personality differs a lot from Ash's other rivals, because instead of looking down on Ash, and thinking he's just a weak loser, like most rivals did untill Ash beat them, he sees Ash as an equal, and appears to even look up to him too.
In the end I'd probably have to go with Gary, since he's, well, Gary >_<