From Bulbapedia, the community-driven Pokémon encyclopedia.
Dr. Mason (Japanese: Ｄｒ．オーヤマ Dr. Ōyama) is a Pokémon card researcher. He resides in Mason Laboratory, where he teaches people how to play the Pokémon Trading Card Game and coaches practice duels. He was named and modeled after Pokémon Trading Card Game creator Kouichi Ooyama.
In Pokémon Trading Card Game, Dr. Mason gives the player a choice of three decks to start off with. They are the Charmander & Friends Deck, Squirtle & Friends Deck, and Bulbasaur & Friends Deck. He also gives the player 30 additional cards, which are decided by which deck the player picked.
In Pokémon Card GB2: Here Comes Team GR!, he gives the player his or her first deck, which is always the same. In both games, he builds a Challenge Machine for card duels after the player finishes the game.
Artwork from Pokémon Card GB2