Collect all character voices. Go to the Voice Collection. Press SQUARE on any given character.

Seraphic Gate:
To unlock the bonus dungeon Seraphic Gate beat the game and wait until the scenes after the "Fin" screen. It will give you an option to save. Save your game (it puts you at the last save point in Asgard Hill). Now go to the main menu and you will have an option to go to Seraphic Gate. You must use your completed game save to get in.

Angel Slayer:
Successfully complete the Seraphic Gate dungeon by defeating Iseria Queen ten times to get the Angel Slayer.

Book of Riddles:
Successfully complete the Seraphic Gate dungeon by defeating Iseria Queen two to nine times. One book (eight total) will be awarded each time.

Character Art:
Aquire all of a character's voices in the Voice collection, then press Square on that character to see a piece of character art.

Secret Scene:
Wait after the credits finish and you will see a scene involving Bramms and Lezard. This can only be posible if you get the A ending.

Tri-Emblem:
Successfully complete the Seraphic Gate dungeon by defeating Iseria Queen to get the Tri-Emblem.

Unlock items by beating the Seraphic Gate multiple times:
Each time you defeat Iseria Queen in the Seraphic Gate, you receive a chest and 1,500,000 exp for your exp orb. Once you defeat her 10 times and receive the Angel Slayer, all successive playthroughs will result in anther Angel Slayer and the orb exp.

Angel Slayer - Beat the Seraphic Gate 10+ times.
Book of Riddles 1-8 - Beat the Seraphic Gate 9 times (one book each time).
Tri-Emblem - Beat the Seraphic Gate once.