To unlock the 2 hidden characters of each class, you must beat the following levels with an S rank.

Effect - Code

Archer 3 - Strive of The Phantom
Archer 4 - The Den of Lizards
Pirate 3 - Curse of Vanity
Pirate 4 - The Big Search
Thief 3 - Eternal Suffering
Thief 4 - Valleys of The Rotten
Warlock 3 - The Downfall of Xylon
Warlock 4 - The Doom
Warrior 3 - The Forest of Ravens
Warrior 4 - The Secret Hideout