From Bulbapedia, the community-driven Pokémon encyclopedia.
A Colosseum Master (Japanese: コロシアムマスター Colosseum Master) is a type of Trainer that debuted in Generation IV. So far, they have only appeared in Pokémon Battle Revolution.
Colosseum Masters are heads of Colosseums, like Colosseum Leaders. They are distinguishable from Colosseum Leaders by the fact that Colosseum Masters do not cosplay as Pokémon, and that all Colosseum Leaders have the same battle music, as opposed to Colosseum Masters who each have unique battle music that can only be heard while battling that Colosseum Master. Another possible way to distinguish them is that they all use legendary Pokémon in their final match set, something that Leaders (or any other Trainers in the game) do not do.
Colosseum Masters are different because they follow unique character models.
There are only three Trainers known as Colosseum Masters: Joe, Sashay, and Kruger.