From Bulbapedia, the community-driven Pokémon encyclopedia.
Magician's Apprentice is a Trainer class introduced in Generation IV. So far, this Trainer class only appears in Pokémon Battle Revolution.
This class appears in Stargazer Colosseum in the third battle of Sets 4 and 8. They are Trainers who study under Mysterial. This class can also be utilized as a custom character by having a Cool Boy on the Custom Pass dressed in Formal Outfit A.