Like the main series of games, Pokémon Trading Card Game has got a rival who is also travelling across TCG Island in order to collect all of the medals and inherit the Legendary Cards

Encounter #1

When you have obtained your first medal, Ronald will appear and ask you if you are to be aiming to inherit the Legendary cards. When you agree, he will snidely say that he will will be the one to inherit them, and then quickly leave.
Encounter #2
Reward: Promo Jigglypuff

After you have defeated your second club, then you will encounter Ronald in the lobby of the club you just defeated. He will immediately challenge you to a duel. You can only do this once, so if you lose, you can only get the promotional card from the Challenge Cup

I'm Ronald Deck
Pokémon Trainers Energy
Card Quantity
3
2
3
1
3
2
2
1
2
3
2
Card Quantity
1
1
2
1
1
1
2
Card Quantity
9
10
8
Encounter #3
Reward: Promo Mewtwo

After you have defeated your third club, then the Challenge Cup will open. You will encounter Ronald within the lobby of the Challenge Cup and he will say about how he is going to win the special promotional card. When you enter the Challenge Cup, he will be your third opponent.

Invincible Ronald Deck
Pokémon Trainers Energy
Card Quantity
3
2
4
3
3
2
2
2
Card Quantity
2
2
2
2
2
2
2
1
Card Quantity
7
6
7
4
Encounter #4
Reward: Promo Super Energy Retrieval

After you have defeated your fifth club, then you will once again encounter Ronald in the lobby of the club you just defeated. He will immediately challenge you to a duel. Like before, you can only do this once, so if you lose, you can only get the promotional card from the Challenge Cup

Powerful Ronald Deck
Pokémon Trainers Energy
Card Quantity
3
2
2
1
2
1
2
1
2
2
3
Card Quantity
2
1
2
1
1
1
2
1
1
Card Quantity
7
9
7
3
Encounter #5
Reward: Promo Mew

After you have defeated your fifth club, then the Challenge Cup will open again. You will encounter Ronald within the lobby of the Challenge Cup and he will say about how he is going to win the special promotional card. When you enter the Challenge Cup, he will be your third opponent.

Invincible Ronald Deck
Pokémon Trainers Energy
Card Quantity
3
2
4
3
3
2
2
2
Card Quantity
2
2
2
2
2
2
2
1
Card Quantity
7
6
7
4
Encounter #6
Reward: Legendary Cards

After you have deleted the Grand Masters for the first time, you will learn that someone else has also defeated them and is poised to inherit the Legendary Cards. In order to work out who is worthy of them, you have to challenge them. As you would expect, it is your rival, Ronald..

Legendary Ronald Deck
Pokémon Trainers Energy
Card Quantity
1
2
1
1
1
1
2
4
4
3
2
Card Quantity
4
3
1
2
2
1
Card Quantity
20
4
Encounter #7

After defeating Ronald in the Grand Masters competition, you will see him at Mr. Ishihara's house in the north-west of the island. When you talk to him, a prompt comes up suggesting a duel, but with both options, you end up not being able to duel against him. You will, however, be able to face him at random whenever the Challenge Cup opens.