Iwashimizu

Rocky is a character of the day who appeared in Training Wrecks (AG096).

Ash and his friends stumbled upon Rocky and his Muscle Island fitness camp while they waited for their Mossdeep City ferry service. His gym is designed to help Trainers work on improving their strength, which inspires their Pokémon to do the same and thereby become stronger in battle. He invited the group to join, and the boys all agreed, and May eventually took to pedaling on a exercise bike.

Rocky is particularly competitive, with another one of his hobbies being battling. Rocky challenged Ash to a Double Battle, and as per camp rules Ash's friends were not allowed to watch the match and so they continued their training on Muscle Island. Rocky, using his Walrein and Quagsire, was able to get the upper hand in his battle against Ash by having the half-Ground Quagsire block all of Pikachu's Electric moves and having his Walrein use the increasingly powerful Ice Ball attack. However, Ash was able to deduce that the speed of the Ice Ball slowed down every turn. Using it to his advantage, he was able to defeat Rocky. Rocky complimented Ash's strategy, admitting that he underestimated the strength of Trainers outside of Muscle Island and would consider going on his own journey. Rocky then pointed out the next ferry to Mossdeep City, so the group went on board.

(Source: Bulbapedia)