Extra Menu
After completing the game and saving your clear data, if you go back to the Title Screen, you'll notice a new Extra option. In here, you can view all the lists you could ingame such as a Monster List, Blade List, and so on. You can also listen to the music of the game and hear the Japanese VAs for some of the characters in the CAST section.
True ending
Successfully complete all quests to unlock the hidden "Truth in Darkness" quest at the board at the Guild. Complete the "Truth in Darkness" quest, then create the Shadow Gem by using the item synthesis on Ruby Prism (obtained from item synthesis on Aroma Material, Brilliant Stone, Dunkelhite and Dragon Tongue) and Broken Gem (obtained during the "Truth in Darkness" quest). Then defeat the final Boss, Uroboros.