From Bulbapedia, the community-driven Pokémon encyclopedia.
A Supertrainer (Japanese: スーパートレーナー Super Trainer) is a type of Pokémon Trainer that first debuted in the Generation III games, appearing in Pokémon Colosseum.
In Colosseum, Supertrainers largely appeared only in the Battle Mode, where they often were the final opponents in Colosseum battles. In Tower Colosseum and Orre Colosseum, their teams often included Legendary Pokémon such as Groudon and Kyogre. Their design also appears in many of the Area Leaders on Mt. Battle.
In Pokémon XD: Gale of Darkness, Supertrainers returned; however, they were greatly reduced in power from Colosseum. They again appeared on Mt. Battle, and also appeared often in Orre Colosseum, but they usually do not use any Legendary Pokémon.