Complete the game and watch the credits the entire way through. You can than make a new save. After that you will be able to select the Bitter difficulty along with your old equipment.
Complete the game and save when prompted. You can then start a new game session and keep the items from your previous game.
To unlock the minigame "Pure White Giant Glastonbury", beat the Rank 04 battle. Go back to the motel and select your TV. The game is playable from there
The Real Ending:
Complete the game with all beam katanas obtained. If you do not have them all by the end of the game, return to Santa Destroy, get the katanas, and return to a bathroom to view the real ending.
Unlock Shirt 00:
Beat the game and you will unlock Shirt 00.