• English / United States: Mateo
• Japanese / Japan: イサオ
• Japanese (Romanized) / Japan: Isao
• Japanese (TL) / Japan: Isao

Mateo runs a glass and crystal shop on Sunburst lsland. His little sister Marissa lives with him and helps him run the shop. When Ash Ketchum and his friends met him, he wasn't feeling very well and didn't want to make glass anymore which was hurting their business. The local shopkeepers were complaining about him because he was hurting their business by not stocking any merchandise which made their stores look bad.

Mateo explained that his creations just didn't have the spark that his Grandpa's Pokémon statues had as they always looked like they might come alive. Mateo said he just needed some kind of inspiration and he hoped that capturing the Crystal Onix would be that spark.

Mateo told Ash Ketchum that he had looked for the Crystal Onix all over the island but he couldn't find it anywhere. With the help of Tracey's Marill, Mateo was able to find the Crystal Onix in a cave in a remote part of Sunburst Island. He battle it but after he knocked it out felt something strange for a minute when he looked at the Onix. He explained that the feeling was hard to describe but he gained the inspiration he need to start working again on his Pokémon statues. As thanks for helping him, Mateo made Ash Ketchum a small figure of Pikachu.