Keys & Potions
Keys allow your fighters to access special maps. Keys are tradable items that can be discovered by defeating special NPCs on the battlefield. Key Items are used from the item menu (the same as Maps and Chests), and then become permanently stored in your Keychain.
XP Potions are consumable items that let you to give bonus experience points to a fighter. They are particularly useful when you've recruited a new fighter and need him to catch up with the rest of your team.