Completion Bonuses:
Beat the game to unlock the Premium Box, Adventure Review mode, Battle Review mode, and the Movie Viewer.

Master Fighter mode:
Get S-rank in all 50 Battle Review missions.

Hidden Boss Fight:
Finish all side-quests and talk to the boss who appears in front of the movie theater.

Easy mode:
Intentionally fail three times and the game will prompt you to switch to easy mode.