Knights are very strong party members, they can equip the strongest weapons and armor and will defend other critical party members, but they lack in the areas of speed and magic.
Monks attack with bare fists for massive damage and they will counterattack, but they can't equip heavy armor (despite a high Stamina, this leaves them vulnerable) and they are tied for the worst magic rating.
Thieves excel at one thing in particular: speed. This, accompanied by their default ability to steal, makes them a worthy party member. They can also dash four times as fast as a normal character. This is only an asset when you really know where you are going. They are, however, pretty weak overall.
White Mages are an essential! Despite terrible scores in the areas of defense and strength they more than make up for it with their healing White Magic. Besides, they won't usually be attacking in critical battles.
Black Mages have the privilege of being the weakest party member that there is. Still, they are the only characters that can take advantage of the enemies elemental weaknesses via their tremendous Black Magic!
Blue Mages are average mages, but they are very versatile once they learn all of their magics. Blue Magic, for those of you who are in the dark, is magic acquired through an enemy's attack. It takes a lot of effort to get all of the Blue spells, but among the Blue Magics you will find are some of the most useful and unique spells that the game has to offer.