Beat the game and start a new game with the cleared data. This time through you will have the Demonic Compendium, no level limit on fusions and summons, and different costumes. You can also change to first person view by pressing SELECT.
Debug-Mode
While in a terminal, swap Nocturne for another PS2 game. Go out of the terminal and when the debug screen pops up, switch Notcurne back.