From Bulbapedia, the community-driven Pokémon encyclopedia.
A Colosseum Leader (Japanese: コロシアムリーダー Colosseum Leader) is a type of Trainer that debuted in Generation IV. So far, they have only appeared in Pokémon Battle Revolution.
Colosseum Leaders are heads of Colosseums, like Colosseum Masters. They are distinguishable from Colosseum Masters by the fact that all Colosseum Leaders cosplay as Pokémon and have the same battle music between them, as opposed to Colosseum Masters, who each have their own individual battle music and dress more professionally.
There are only six Trainers known as Colosseum Leaders: Taylor, Marina, Rosie, Voldon, Terrell, and Dusty.
Despite dressing up as a Pokémon, most Colosseum Masters (short of Taylor, who has a Pachirisu outfit) do not use the Pokémon they dress up as the first time they are faced. Some will use the pre-evolutions of their outfit, while Terrell and Marina do not use their outfit Pokémon (Groudon and Kyogre, respectively) until the Colosseum rank is at least level 7.