From Bulbapedia, the community-driven Pokémon encyclopedia.
A Colosseum Master is a type of Trainer that debuted in Generation IV. So far, they have only appeared in Pokémon Battle Revolution.
Colosseum Master are heads of Colosseums, like Colosseum Leaders. They are distinguishable from Colosseum Leaders by the fact that Colosseum Masters don't 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.
There are only 3 Trainers known as Colosseum Masters: Joe, Sashay, and Kruger.