From Bulbapedia, the community-driven Pokémon encyclopedia.
Jimmy (Japanese: ジミー Jimmy) is a character of the day who appeared in Battling at Full Volume!.
Jimmy, a guitarist, came across Team Rocket and heard about a Pikachu. He then came across the campsite where Ash and his friends were staying. He alerted Serena and her Pokemon, along with Ash's Pikachu, by playing his electric guitar loudly. He asked Serena about Ash and wanted to challenge him and his Pikachu to a Pokémon battle. Jimmy said that he wanted to be the strongest Pikachu Trainer, with his partner Pokémon Pikachu, who he nicknamed Spike (Japanese: トンガリ Tongari).
Since Ash had a cold at the time and needed to rest, Serena decided to disguise herself as Ash by wearing his clothes and speaking in a deeper voice. Serena then battled Jimmy, but the match became interrupted when Clemont and Bonnie returned to the site after purchasing medicine for Ash. The battle then continued until Team Rocket kidnapped Ash's Pikachu and Spike.
After Ash recovered thanks to the nursing that Clemont, Bonnie, Serena, and her Pokémon gave Ash, he stopped Team Rocket from escaping with the two Pikachu. Ash and Serena then teamed up to rescue the Pikachu from Team Rocket. Then, Jimmy and Ash commanded their Pokémon to both use Thunderbolt, which sent Team Rocket, their Pokémon, and their getaway truck blasting off into the sky. After Team Rocket was dealt with, Jimmy complimented Serena on her battling skills. Then, Ash challenged Jimmy to a battle, but since Ash became hungry, the battle was postponed until after they all had dinner together.
This listing is of Jimmy's known Pokémon in the anime: