For 100 % Completion in Kingdom Hearts, you must do everything there is to do (such as: find all 99 Dalmatians, do all the trinities, unlock all spells and their upgrades, lock all keyholes- even 100 Acre Wood, etc.)

Your REWARD: If you completed the game before, you will be able to get a different ending artwork when you finish and it says "end." Also, you will see the Secret Movie. (It is a preview of Kingdom Hearts II. WORTH IT, I guarantee it. Though it isn't that long and you're left going "what?" It's worth it.)