The name of the job that thief get changed is very cool - assassin, who can use Katar with high Attack Spd and Cri as weapons.
Melee, Physical, and High Burst
Job Change Information
You can change jobs to an Assassin at the Assassin Guild, located East of Morroc. Apply to the Guildsman inside the Guild and talk to each test administrator accordingly to begin the test. The Assassin test strengthens your swiftness and judgement. You will be able to change jobs once the Guildmaster has completed his review. Requirements are Job Lv 40 or higher.
01: Go inside Assassin Guild entrance.
02: The Guildsman will send you to the job change.
03: Quickly seize the targets.
04: Complete the Guildmaster's review