Meet the conditions to unlock new characters, which may be accessed when selecting a character at the main menu.

Character	How to unlock
Brawler	Kill over 500 enemies.
Cleric	Defeat the Soul Guardian.
Engineer	Obtain a rocket.
Necromancer	Kill 20 skeletons in one game.
Nightblade	Kill a necromancer.
Warrior	Defeat the Skeleton King.
Wizard	Find Merlin's Hat.