To change which person you control on Phantom Island, pick up a Changebook. You can get one by jumping on top of the really big tree from Marona's house. Go to whomever you want to control and press R2 as if you were attacking them. You can use this with the "Unknown Phantom tricks" for getting experience or mana. This allows you to level up your Dungeon Monk, Merchant, Old Man, Old Granny, Healer, etc. without having to risk them in a battle. You can even control the Unknown Phantom itself, which means that if you level him up and let the others hit him after he has been leveled you can get even more experience or mana for your characters and items.