Dragon Slayer & Goddess Pendant
Dragon Slayer - Head to Armgrins' house in Artolia on normal level and go to room to the far left and you will recieve dragon slayer sword that Armgrin recieved from his father when he was alive.
Goddess Pendant - Head to where the cutscene with Millia & Llewilyn happened in Lassen. It is under the trees to the very far right.

Character art
Unlock all entries for a character at the "Voice Collection" screen. Highlight that character, then press Square to view their art.

Hidden scene
Complete the game with the "A" ending. After the credits end, there will be a scene involving Bramms and Lezard.
Angel Slayer
Complete the Seraphic Gate dungeon by defeating Iseria Queen ten times to get the Angel Slayer.

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.
UnlockableHow to Unlock
Angel SlayerBeat the Seraphic Gate 10+ times.
Book of Riddles 1-8Beat the Seraphic Gate 9 times (one book each time).
Tri-EmblemBeat the Seraphic Gate once.

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

Seraphic Gate
Complete the game and save when prompted after the credits. A new option will be unlocked at the main menu that allows access to the Seraphic Gate dungeon through the cleared saved game file.

Secrets
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
Complete the Seraphic Gate dungeon by defeating Iseria Queen to get the Tri-Emblem.

Unlockables
Character Artwork
In the Voice Collection, if you have acquired all of a certain character's
voices, pressing Square on that character will bring up artwork for the
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.