To unlock character art, aquire all of a character's voices in the Voice collection, then press Square on that character.
To unlock Seraphic Gate, complete the game and save after the Fin screen. You will now be able to access Seraphic Gate through the main menu.
To get Seraphic Gate items, beat Iseria Queen:
Tri-Emblem: beat the Seraphic Gate once
Book of Riddles 1-8: beat the Seraphic Gate nine times
Angel Slayer: beat the Seraphic Gate ten times