Yukinari Okido (Professor Samuel Oak in the English and Italian version, Professor Carvalho in the Brazilian version), is a Pokémon researcher, and generally considered the best of his kind. As such, his role in the Pokémon games and anime is that of a mentor to young Pokémon Trainers, a source of information and an occasional plot device. Amongst other things, he is credited with inventing the Pokédex. Although he is an expert on all matters relating to Pokémon, Professor Oak specializes in Pokémon behavioural science.
In the pokemon 4ever movie, it is revealed that, as a kid, Samuel met a Celebi and traveled through time with him. He then came to the present time and became Ash's friend. After the defeat of Viscious, he travels back to his time.