From Bulbapedia, the community-driven Pokémon encyclopedia.
A Grand Duke (Japanese: グランデューク Grand Duke) is a type of Pokémon Trainer that debuted in Generation VI. This Trainer class is a title of nobility, granted in the Battle Chateau, and Trainers with this class only appear there. Grand Duke is the highest rank of nobility for males, ranking above Duke. They are the male counterparts of the Grand Duchess class.
The male player character may earn the title of Grand Duke by defeating most of the Trainers in the Battle Chateau. In game, no non-player character possesses this Trainer class.
In the anime
Grant being promoted to Grand Duke in the anime
In Breaking Titles at the Chateau!, Grant defeated Viola in a battle at the Battle Chateau, earning himself the title of Grand Duke as his reward.
In the anime, Grand Dukes are shown wearing purple capes as a distinction to the other titles
In other languages