Once you've snagged a Shadow Pokémon, it's not immediately ready for full use. Shadow Pokémon are corrupted by Cipher, and their true potential is locked away. To unlock their power and remove their negative effects, you must undergo the "Purification" process. This involves raising the Pokémon's "Heart Gauge," a unique mechanic in Pokémon Colosseum that represents the Pokémon's emotional state and willingness to trust you.
The Heart Gauge fills up through various activities. The most effective methods include winning battles with the Shadow Pokémon in your party, walking with it (the more steps taken, the better), and using the "Call" command in battle, which strengthens the Pokémon's resolve. Conversely, having the Shadow Pokémon faint in battle or using it in battles where it's at a significant type disadvantage will lower its Heart Gauge. Once the Heart Gauge is full, the Pokémon will undergo purification, often accompanied by a special animation. Purified Pokémon gain access to their original move sets, learn new powerful moves, and their stats are significantly boosted. This process is essential for building a strong and reliable team.