From Bulbapedia, the community-driven Pokémon encyclopedia.
Seymour (Japanese: リカオ Rikao) is a character of the day who appeared in Clefairy and the Moon Stone. He is a scientist who lives on Mt. Moon with the various Clefairy and Clefable.
Ash and his friends saw Seymour being attacked by a group of Zubat. Pikachu saved Seymour by scaring the Zubat away, which Seymour was thankful for. He explained to Ash and his friends that he was on Mt. Moon in order to investigate the reason why Pokémon here were behaving strangely. After discovering it was Team Rocket's scheme, the trio was sent away, and Ash's 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 of a giant Moon Stone, Seymour decided to live on Mt. Moon with the Clefairy and Clefable.
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 理科 rika, which means "science".
He reappeared in flashbacks in the next episode and Bye Bye Butterfree.
Seymour often enjoys talking in rhymes, such as "we've studied its fragments for many an hour, and discovered it increases a Pokémon's power."