Available MagicWhite Magic LV. 1–3 (cannot be used during combat)
The Mystic Knight is a strange job, not quite like any other in the series. At first they are useless, because you don't have any gear to give them. Once you get The Invincible, you can finally use Mystic Knights, and you'll have to, since their Dark-elemental weapons are the only safe way to physically attack Dividing enemies like those in the Cave of Shadows. Mystic Knights are powerful front-line fighters, and they have the odd ability to cast low-level White Magic (notably including Cure and Cura) outside of combat.
As a Dark Knight, Cecil is a no-frills fighter. His Dark Swords can't do much against certain Undead, but other than that he can dish out plenty of damage with his normal attacks. His Dark ability sacrifices 1/8th of his max HP in order to deal damage to all enemies. It isn't much damage, but it can be helpful for quickly taking out groups of weak foes. Don't overdo it, though, as the damage to Cecil adds up quickly.