I agree with Brendino that the rivals were pretty insignificant in this game, which is weird since they were so important the last two games. However, I guess they were going for a change, and when you compare it to the games that came after it, RSE are really the only main series games that don't focus on a rival too heavily. And that's fine with me.
As for the rivals themselves, I was fine with Brendan's personality. He wasn't an ass, but still obviously wanted to prove himself against you, so I thought that was nice. They made May a bit too shy for my liking though. I liked Wally and as much as I want to consider him a rival, it's really difficult when you don't battle him but twice so I don't really feel a rivalry with him like I do Brendan and May. Wally feels more like a colleague, if anything. Though that word might be too strong for a relationship between 10 year olds haha.
I'm going to say I like Brendan as a rival (more so than May, since like I said, too shy imo), since he felt more like one than Wally did (however UGH they should have gotten their starter to the final evo!!) and I see Wally as more of a friend/colleague.