To get Celebi in Pokémon Colosseum, you need to use the Japanese bonus disc for the game in conjuncture with a supported Game Boy Advance Pokémon game. You need the Japanese bonus disc for GameCube, all 48 Shadow Pokémon, a GBA console and a GBA to GameCube cable link.
- Set up the GBA
Load one of the following Pokémon games into the GBA: Sapphire, Fire Red, Leaf Green, Ruby or Emerald. Make sure that you have beaten the Elite Four. Go to any Pokémon Center in the game. Make sure that you have no more than 5 Pokémon in your party. Save the game.
- Insert the bonus disc into the GameCube
Make sure that you have no more than 5 Pokémon in your Pokémon Colosseum party. Insert the Japanese bonus disc into the GameCube, and turn the console on. Bypass the various preview screens, and select Pokémon Colosseum. Make sure that you have purified all 48 Shadow Pokémon, and go to the Sacred Shrine. Allow the cut-scene to play through.
- Link the GBA
When Eagun asks, "Does your friend want to receive the blessing of Celebi, too?", use the cable link to connect the GBA to the GameCube. Celebi transfers to your GBA party automatically. Trade Celebi from the party in the GBA game to the party in Pokémon Colosseum.