Your rival in this game is named Ronald. You will meet him at various points in the game based on your progression. The first time you meet him will be when you enter the lobby of your first Pokémon Club. This is just his introduction though, there isn't a battle at this point.
Two Master Medals[edit]
After winning your second Master Medal, as you enter the lobby of the Pokémon Club you won it from, Ronald will appear and challenge you. He will be using his I'm Ronald Deck.
Strategy[edit]
Ronald's deck isn't very threatening unless he's able to pull off evolving some of his Pokémon. This can be difficult for him due to having several evolution lines in the deck. Even then, he has no Blastoise or Charizard to evolve into, seriously hindering the effectiveness of his deck.
Cubone can stall with its Snivel attack to reduce how much damage you can do to it, but Ronald seems to be more aggressive with it and will use Rage if he has enough energy on it instead. If it evolves into Marowak it is only dangerous if it gets heads flips on its Bonemerang attack where it flips two coins for 30 damage each.
Lapras can be a threat if your deck is Fire-focused as its Water Gun can be pretty powerful doing 60 damage because of weakness. As long as you have other Pokémon that can take 30 damage hits for a turn or two it shouldn't be too difficult to take out. Just be ready in case its Confuse Ray gets a head flip and cause the confusion status.
Winning will reward you with the Jigglypuff promotional card.
Five Master Medals[edit]
Once you win your fifth Master Medal you will once again run into Ronald as you're passing through the lobby to the exit. This deck will be the Powerful Ronald Deck.
Strategy[edit]
You'll receive the Super Energy Retrieval promotional card if you win.
Even More[edit]
There are more places you'll find Ronald as well such as the Challenge Hall.
And the Pokémon Dome