I excluded Ronald for being out of place. Unlike player characters, professors, regions and villainous teams, there are few rivals to be had in the non-traditional games. Even if the particular character of Ronald matches the definition well, being to Mark, Mint and Dr Mason as Green is to Red, Leaf and Professor Oak, he seems out of place when added to the list of GameFreak-made rivals.
I should also note that I am tempted to remove Lucas and Dawn, as well. The characters play a role other than that of the player, much like Brendan and May do in the predecessor games; however, in that role they do not so much as battle the player. To a certain extent, Emerald Steven is a more valid candidate for the category, in that he goes from partnering the player to battling them. Furthermore, the characters' dialogue lacks the distinguishing marks of what would be expected from a rival (they make no attempt to compete with the player for PokéDex completion). Simply put, they are mellow throughout the entire game, whereas Brendan and May progressively transformed to the point of losing their hold on the rival title (to Wally).
That said, I feel that their removal would shift the impression of incompatibility that I mentioned in regard to Ronald, over to Brendan and May. Considering that Brendan, May, Wally, Lucas, Dawn and Barry are all referred to as "Pokémon trainers", as opposed to "rivals", the exception to the standard definition may be acceptable (out of these six, Lucas and Dawn are the farthest from Green and Silver, while Barry is the closest). --Unown Lord
| Blue | Silver | Wally | Pearl|
Brendan | May | Ronald