Register someone multiple communication IDs on Mog Net. Send seven messages to another player. Send and receive four messages from Topapa. After reading the fourth one you will get a message giving your a mini-quest. Go to Uoul and talk to Topapa. Go to the cave that is to the north of the other cave. Enter, then defeat the three Bombs to get a piece of crystal. This will unlock the Onion Knight as a job.