From Bulbapedia, the community-driven Pokémon encyclopedia.
Mateo (Japanese: イサオ Isao) is a character of the day in the Pokémon anime who appeared in The Crystal Onix.
Mateo and his younger sister Marissa live on Sunburst Island in the Orange Archipelago. Mateo owns a glass shop (inspired by his grandfather), but his products are lower quality than the other glass makers on the island. One shop owner yelled at Marissa claiming her brother was ruining the glass business on the island.
Mateo's ultimate goal was to capture the legendary Crystal Onix. With the help of Tracey's Venonat and Marill, he found the Pokémon in a cave. Using his Pokémon, he weakened it, but out of nowhere, decided not to capture it. He told Ash and his friends that just seeing the Crystal Onix had inspired him.
When he returned to his glass shop, just like his grandpa, Mateo was able to make much better quality glass pieces and soon his business greatly improved. As a memento of thanks, Mateo gave Ash a Pikachu glass figurine.