It's not enough to just meet the requirements for each humanoid class. You must pass the bill associated for each one as well.

Class	How to unlock
Android	Must beat the main story and be on a 2nd cycle or higher.
Archer	Healer and Valkyrie level 15.
Armor Knight	Warrior level 20.
Battle Suit	Warrior and Armor Knight level 30 and clear Episode 8.
Beast Master	Warrior and Valkyrie level 20.
Bouncer	Clear Episode 7 or Armor Knight level 100.
Gunner	Thief and Skull level 15.
Lady Samurai	Valkyrie and Archer level 30.
Magic Knight	Warrior or Valkyrie and Mage or Skull level 30.
Masked Hero	Thief and Warrior level 25.
Ninja	Thief and Skull level 30.
Onmyo Monk	Sorcerer and Ninja level 30.
Professor	Healer and Onmyo Monk level 20.
Sorcerer	Skull and Witch level 15.