From Bulbapedia, the community-driven Pokémon encyclopedia.
Seymour (Japanese: リカオ Rikao) is a scientist who lives on Mt. Moon with the various Clefairy and Clefable.
Seymour appeared as a character of the day in the episode Clefairy and the Moon Stone. His Japanese voice actor is Tetsuya Iwanaga and his English voice actor is Maddie Blaustein.
In Clefairy and the Moon Stone, Ash and his friends saw Seymour getting attacked by a group of Zubat. Pikachu saved Seymour by scaring off the Zubat, and he was thankful for the group for saving "his life". Seymour explained to Ash and his friends that he was at Mt. Moon investigating why the Pokémon there were behaving strangely, because they were weak and sick. After discovering it was a Team Rocket scheme, they were sent away, and the group began to follow a Clefairy.
Seymour revealed his theory that the Clefairy came from space in a shuttle created from a Moon Stone. After witnessing several Clefairy evolve into Clefable with fragments from a giant Moon Stone, Seymour decided to live at Mt. Moon with the Clefairy.
In the Japanese version, Seymour is from Pewter City and works for the Pewter Museum of Science. Instead of reciting poetry, he is enchanted by the romantic aspects of the moon. His name is a play on rika, which means science.