If you find any type of crest, most likely the description will read "for <class name> level 10 and up". Give it to the type of class it mentions. For example, give a Knight Crest "for knights level 10 and up" to any knight you have in your group. Once they reach level 10 or higher, have them "use" it. It will show up in ther inventory while in battle. This will initiate a process in which you can choose how they change classes into a more powerful character. Once you choose, they will be transformed into a better fighter, and you can continues leveling them up until they max out and are the most powerful they can be (usually at level 20). This same effect can happen on certain characters with other items. Give Guiding Rings to magicians and troubadours. There are some other special items that can be used on any character, such as Angelic Robes (raises maximum health), Special Rings (raises strength or speed), and the Ocean Crest for thieves and pirates.