Note that just having the mastery isn't enough you'll then have to petition the Dark Assembly with the appropriate bill to be able to create them.

Class	How to unlock
Archer	Have someone with a level 5 bow mastery.
Beast Tamer	Successfully capture a monster.
Chaos Soldier	Finish the game once.
Druid	Perform a 500+ Geo Combo.
Female Ninja	Have someone with a level 5 fist and staff mastery.
Gunner	Have someone with a level 5 gun mastery.
Heavy Knight	Have someone with a level 3 spear mastery.
Magic Knight	Have someone with a level 3 sword and staff mastery.
Male Ninja	Have someone with a level 5 fist and sword mastery.
Samurai	Have someone with a level 5 sword mastery.
Sinner	Someone in your party becomes a criminal.