From Bulbapedia, the community-driven Pokémon encyclopedia.
A.J. (Japanese: アキラ Akira) is a Pokémon Trainer in his mid-teens who lives between Cerulean City and Vermilion City. Other trainers refer to him as a "Wild Pokémon Trainer", although what this is meant to imply is never made clear. His dream is to become a Pokémon Master with the help of his favorite Pokémon, Sandshrew. He had a gym, but it was an unofficial gym not sponsored by the Pokémon League and did not have a related badge.
In the anime
A.J. was the character of the day in The Path to the Pokémon League. His Japanese voice actor is Nobuyuki Hiyama and his English voice actor is Maddie Blaustein.
In this episode, Ash, Misty, and Brock were on the way to Vermilion City, when they heard about a local trainer who was undefeated. Ash, eager to challenge this trainer, went to A.J.'s gym and saw a sign above it announcing that A.J. had been undefeated through 98 matches. A.J. returned to the Gym after catching some wild Pokémon. Ash challenged him to battle the number 99.
Ash begins to say about a person with 98 victories but without a Gym Badge. A.J. says that he wasn't planning on challenging a Gym Leader until he had 100 wins in a row. Ash bragged about the two badges he had at the time just with ten victories, and A.J. insulted him and said he must've gotten them from weak Gym Leaders, which angered Brock and Misty. A.J. defeated Ash with his Sandshrew without much effort, causing Ash to complain and accuse A.J. of cheating.
As the group was about to leave, they looked inside the Gym, and they saw A.J. using a whip to send his Sandshrew diving into a pool. Ash attacked A.J., thinking that he was torturing his Pokémon. A.J. then explained that he was strengthening his Sandshrew's resistance against water. Sandshrew and all of A.J.'s other Pokémon were wearing Macho Braces to intensify their strength. He also had all of his Pokémon exercise is rigorous routines. Ash complained about A.J. treating his Pokémon too harshly, and asks Brock and Misty to back him up. However, instead Brock compliments A.J. on the specialized food he prepares for his Pokémon.
Jessie, James, and Meowth then tried to steal Pikachu, but ended up accidentally taking Sandshrew instead. Not knowing what had happened, Ash guessed that Sandshrew had ran away, and offered A.J.'s other Pokémon spots on his team. They ignored him. Sandshrew escaped from Team Rocket by using Dig, accidentally bringing Meowth along with him. After A.J.'s Sandshrew came back, he challenged Team Rocket to a battle, and his Sandshrew defeated Team Rocket, winning A.J. the final victory needed to set off and collect Gym Badges.
A.J. makes another appearance in a flashback sequence of the Orange Islands opening song Pokémon World. He and his Sandshrew appear with a group of other Trainers Ash encountered during his first trip through Kanto.
This listing is of A.J.'s Pokémon in the anime.
In the Electric Tale of Pikachu manga
A.J. also makes an appearance in the Electric Tale of Pikachu manga. A.J, along with Samurai and Ash, wanted to find the Hidden Village run by Melanie. Like with some of the other character redesigns, A.J. looks more like Brock's age unlike in the anime.
In the games
In Pokémon Yellow, a game based on the anime, there's a Youngster on Route 9. This trainer solely uses a Level 24 Sandshrew that has the move Fissure, much like A.J.'s Sandshrew in the anime. After the player wins the battle, he says, "I'll restart my 100-win streak with Sandshrew," a reference to the fact that A.J. won 100 battles. This trainer may be A.J., or a reference to him. In Pokémon Red and Blue, games made before the anime, this trainer had a Growlithe and a Charmander, and just said "Keep walking!", after being defeated.