Unlock secret characters by playing Arcade mode repeatedly and collecting enough points to spend in Shop.

Character	How to unlock
Hakushon Daimaou	Clear Arcade mode once, then purchase him for 1000 points
Ippatsuman	Clear Arcade mode thrice, then purchase him for 3000 points
Saki Omokane	Clear Arcade mode once, then purchase her for 1000 points
Viewtiful Joe	Clear Arcade mode thrice, then purchase him for 3000 points