Wally is a rival in the R/S/E games, you first see him in the Petalburg City Gym asking Norman for help catching a Pokemon. After the player character watches him catch a Ralts, you won't see him again until Mauville City. After that, you will not see him again until Victory where he is either your first or last opponent there. In the third generation games, obviously, Wally has a Gardevoir, but in the remakes, Wally evolves his Ralts into Gallade instead, and can mega evolve it.
What did you think of Wally? I think he wasn't too bad of a rival, despite him barely being in the game.
In my opinion, Wally was very easy to defeat. Although, his team was pretty good. In the remakes of gen 3, I would have preferred if Wally had evolved his Magnezone, and maybe put Air Balloon on it, making it harder to hit it with Mega Swampert. I destroyed his team with just Mega Swampert and Swellow. Throughout the duration of the game, he seemed to be the weak, nervous character that would never amount to anything. And then he seemed to change like Neville Longbottom in Harry Potter. He grew older in a way. He seemed to push things past him and managed to get all the way to Victory Road to face you. He sure kept his promise that he made to you in Mauville City! All in all, i liked Wally as a rival, but i wish he was more present throughout the game.