Beat 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.
Unlock all entries for a character at the "Voice Collection" screen. Highlight that character, then press to view their art.
Beat the Seraphic Gate dungeon by defeating Iseria Queen to get the Tri-Emblem.
Books of Riddles:
Beat Iseria Queen again two to nine times. One book (eight total) will be awarded each time.
Beat Iseria Queen ten times to get the Angel Slayer.
Beat the game with the "A" ending. After the credits end, there will be a scene involving Bramms and Lezard.
Thanks to Revolution Readers ALAX and Loki!