I'll have to elaborate later, but here's a quick rundown of my idea for now:

Warrior - Squire from a kingdom tasked to to restore the crystal, the Knight he served scoffed at it and so the Squire stole the crystal and took his place.

White Mage - Holy Order who guards the Crystals, it is they who tasked the various kingdoms to send their champions to restore the crystal. White Mage is an apprentice who was foreseen by prophecy to be the one to restore the crystal.

Black Mage - From an order of dark forces who wish to use the crystals power for their own gains in arcane knowledge, they sent one of their own to restore the crystal so they may all benefit from it's power.

Thief - Stole the crystal from a warrior tasked to restore them, thinking it only a trinket, they take it to the meeting place in the guise of the man they stole it from and instead get swept up in the quest instead of a reward.

Monk - A man filled with turmoil who took refuge with the monks to calm his heart, the journey of the crystal is given to him in order to complete his training.

Red Mage - An adventurer and world traveler who comes across a warrior summoned to deliver the crystal and restore it. The two are attacked by monsters and the hero falls, but tasks the Red Mage to complete the journey for him. Having nothing better to do, they go along.