Unlock Seraphic Gate:
The bonus dungeon 'Seraphic Gate' which is a seperate part of the game is unlocked when you have successfully completed the game 10 times. It can also be unlocked if you 'Save' in the final 'Save Point' of the game just before the final boss. Reset the game and there will be a 'Seraphic Gate' option available at the Main menu.

Unlock Character Art:
When you have 100% of a character's voice go to the 'Voice Collection' option under the 'Sound' menu and highlight that character's name and press Square.

Unlock Alternate Ending:
When you have successfully completed the game with an 'A' rank wait for the credits to end and you view a scene with Brahms and Lezard talking about their future plans.