Complete Christmas NiGHTS with Claris and Elliot to unlock the "Nightopian Collection" at the main menu. This option displays the Nightopians, Mepians, Nightopian, and Mepian eggs in each level; and the mood of the Nightopians and Mepians in that level.
Press the following at the title screen: LEFT, RIGHT, SQUARE, CIRCLE, X, TRIANGLE, R1, R2, DOWN, UP, L1, L2. If done correctly, a message will appear telling you everything's unlocked. Note: you can't acquire trophies or upload your highscore.