From Bulbapedia, the community-driven Pokémon encyclopedia.
Marissa (Japanese: マサミ Masami) is a character of the day in the anime who appeared alongside her older brother Mateo in The Crystal Onix.
Marissa had thrown a bottle in the ocean with a letter in it asking if anyone knew about the Crystal Onix, and that, if they had, to come see her on Sunburst Island. Ash and his friends happened to come across the letter and headed for the island. When they found her she was being terrorized by one of the other shop owners saying that Mateo should just close up the shop. Unfortunately Ash didn't know anything about the Crystal Onix but he did offer his help.
Marissa is a sweet girl who looks up to her brother and is always willing to help him. Even though Mateo didn't believe in himself at times, she always did.