Ash and his friends first met Seymour after they rescued him from a horde of Zubat at Mt. Moon’s entrance. Seymour explained that he was visiting Mt. Moon to investigate why its wild Pokémon were behaving oddly. He directed the group’s attention to some bright lights set up throughout the cave complex and soon discovered that Team Rocket was responsible.
After Team Rocket was sent blasting off, Seymour, Ash, and his friends witnessed several Clefairy evolving into Clefable with fragments of a giant Moon Stone. Seymour decided to live amongst the Clefairy and Clefable with the hopes of visiting their true home and proving his theory to be true: that Clefairy came from space in a shuttle created from a Moon Stone.
He reappeared in Ash’s flashback in Bye Bye Butterfree.
Seymour often speaks in rhymes, while in the Japanese version, he tends to recite poetry and be enchanted by the romantic aspects of the Moon. Seymour is usually depicted as a nervous and neurotic person, but nevertheless is incredibly knowledgeable about his passions, such as extraterrestrial Pokémon.