Professor Samuel Oak is a Pokémon researcher who was once a competitive Trainer, and is generally considered the best in his field.
Ultimate Story[]
Professor Oak made his debut in the first Ultimate Story. He was the one that gave Lisa her Cubone. Later on, he helped the girls with their quest to find the Pokemon Stones. It is also revealed that his is a good friend of Professor Utonium, the Powerpuff Girls' dad and creator.
During the beginning it is revealed that he knows more about the Mystics than he let on. He even revealed some secrets that he had always known from Lisa, but keeps quiet about them as he is a guardian of many of these secrets. Being a guardian, he took an oath of secrecy and to never reveal any of those secrets to anyone unless he needed to.
During Ultimate Story 2, it is revealed that he is also a guardian of a massive mystic library which houses the many secrets of the Universe. The library, itself, maintains recording of the many memories of the Great Wizard, the master of the Universe, itself. However, some of those books are empty, an indication of something that the Great Wizard has forgotten.
Professor Oak allowed Fox, Misty, Krystal and Falco to enter the library after they were able to pass a rigorous test. He also reveals to them that he is the last Descendant of the Great Wizard, himself. Being that he has undying loyalty towards the Great Wizard and the Mystics that also serve under him, he only reveals these secrets to others under his ancestor's order.
Professor Oak had been more than helpful throughout the entire US in many different ways.
During the post US4 story, he summoned Obi-wan into the realm where the Great Wizard's current incarnation was to grant him the title of Library assistant, which gained him full permission to access any intel within the Library. However, he can't enter it by himself since he is unable to open portals to the sacred realm, so he would need to ask the professor to allow him access in order for him to enter whenever he needs to.
Fairies of Kalos[]
Link and Zelda were called out on an assignment from Prof. Oak to investigate Kalos and the mysterious Fairy-type Pokémon.