Assassin (Female Selke) - Appears on Hard or Very Hard only, world map, near Desert Ruins, must be in Multiplayer and have an extensive friends list.
Gerald (Male Clavat) - Appears on Hard or Very Hard only, world map near forest, must have 1,000 kills.
Nun (Female Clavat) - After completing Library 3 on Hard, find her in Library 1 and talk to her to unlock a quest. Complete the quest Monster Busters 2 and talk to her
Penguin (Female Lilty) - Appears on any Difficulty, in front of the Library, must be on Very Hard to recruit.
Red Mage (Male Selke) - Appears on any Difficulty, in front of the library, pay 5,000 Gil.