Ghost Sync

After you have finished the game and reloaded your save, teleport back to the entrance of the Sky Castle. Instead of going straight, head to the left. You should find a new entrance available now that will take you to a secret dungeon (Akegarass). You'll see a broken column laying across a gap (looks like a light gray log) that lets you enter. Finish this new dungeon and beat the boss at the end to receive the Gold Spirit Armband. Have Lily equip this then teleport back to Sharl and beat her again. This will get you the true ending.