Weapons: Knives, Boomerangs
The thief is your standard fast but frail class, able to Steal items. Most of the time, your steals are going to be fairly standard, such as potions. But many enemies will also drop things like Heavenly Wrath, and even Phoenix Downs and other useful items! Not to mention once you unlock boomerangs, you can put the thief in the back row to deal full damage without taking full damage. There are also several dungeons with locked doors. If you don’t buy Magic Keys , you can simply have a thief lead your party to unlock them.
Also, a job level 71+ Thief is the only way to acquire Gungnir from the boss Odin, the second most powerful Lance in the game (the most powerful, the Magic Lance, is exclusive to the Blacksmith sidequest). So if you like Dragoons, consider investing in a Thief so you can get some great equipment for them. After the last crystal, the Ninja will be your choice of upgrade, unless you need the ability to steal.