From Bulbapedia, the community-driven Pokémon encyclopedia.
Gilbert (Japanese: ハジメ Hajime) is a one-time character and a young boy who is the son of the mayor of Pallet Town. He appears in Journey to the Starting Line!.
He wanted a starter Pokémon from Hoenn like Torchic because they were "special", or a Pikachu like Ash Ketchum's, who he seemed to have heard good things about. At the beginning of the episode, he didn't really understand the point in having Pokémon other than for protection. He didn't want the hassle of raising them and was quite spoiled and selfish, making lots of demands for evolved and rare Pokémon like Raichu.
However, while accompanying Professor Oak and the others during the episode trying to recapture the escaped baby Bulbasaur, Charmander, and Squirtle, he begins to learn a lot about the feelings of Pokémon and grows to understand them. Gilbert eventually chose the Bulbasaur he'd helped save from a group of Primeape after he was impressed by the strength of Ash's Bulbasaur at driving them back with a Solar Beam.