At the 'Extras' menu of the DVD that is included with the game press Right and select any of the options except for the DVD credits. Each Area contains two rare items only obtained in it's corresponding dungeon.
Unlock Kokoro (Chest) + Rogue Fragment (Gott Statue): Sigma Server - Entwined Prejudiced Chaos
Unlock Lady Killers (Chest) + Boxed Lunch (Gott Statue): Sigma Server - Greedy Gambler's Drift
Unlock Lady Shoes (Chest) + Scent of Gero (Gott Statue): Sigma Server - Ancient Destroyer's Battlefield