Complete the game and load your cleared saved game file to start a new game with NPCs such as Kee Keema, Meena, Wataru, Mitsuru, and Kutz. Additionally, enter the "Other" menu, and select "Monster Album". You can view details on any enemy by pressing X while they are highlighted.
After you defeat the three sets of twins at the Bandit's Hideout, Leynart will capture the thieves and reward your party with a feast. He will then bid you farewell and give you a bag of "gems" that they had in possession. Yuno will take the bag and ask which of the four stones you would like: Red, Blue, Purple, or Green. Whichever stone you choose will be socketed into your Traveler's Sword, so choose wisely. The Red Stone gives resistance to fire-based attacks and grants you fire power within the sword. The Blue Stone does the same with ice/water, the Purple Stone with lightning, and the Green Stone with wind. Your sword skills will depend upon your choice.