For each class there are 2 extra hero skins that can be unlocked. In order to unlock these, you must get an "S" rank on the stage listed below:

Extra Archer 1 - Stage 6
Extra Archer 2 - Stage 36
Extra Pirate 1 - Stage 13
Extra Pirate 2 - Stage 29
Extra Thief 1 - Stage 15
Extra Thief 2 - Stage 24
Extra Warlock 1 - Stage 10
Extra Warlock 2 - Stage 32
Extra Warrior 1 - Stage 2
Extra Warrior 2 - Stage 40