Back in the original Famicom FFIII, Onion Swordsman was the first job you start as (unlike Freelance from the DS version), but you can play this job in the Nintendo DS version as well by unlocking it. Onion Swordsman is one of the most powerful jobs with the Onion Equipment + past level 90's.

How to obtain:
1)Register someone elses's (or multiple) communication ID under Mog Net.
2)Send message to another PC 7 times
3)Send and receive 4 messages from Topapa, on the fourth one you will receive a message to do a mini-quest.
4)Go to Uoul (First town in the game)
5)Talk to Topapa
6)Go to the cave up north of the cave
7)At the drop hole, you will have an event and then will be proceeded to fight 3 bombs
8)After the battle, you will obtain a piece of crystal, which will unlock Onion Swordsman