Successfully complete the game on any difficulty setting with at least one character, then save the game to unlock the monsters you encountered in your previous game in duel mode.
Cheat mode
At the PCCW Japan logo, hold L1 and rotate the Right Analog-stick until you hear a sound. All duel mode characters, Aria/Iria in story mode, and full attack mode will be unlocked.
Play as Aria/Iria
Successfully complete the game with all four staring characters under the normal or higher difficulty setting to unlock Aria/Iria in story mode.