Fight as Amakusa Shiro Tokisada (Samurai Shodown 1):
Use any character to successfully complete story mode and save the game. Enter versus mode, highlight the "?" on the character selection screen, then hold L1 + L2 + R1 + R2 and press Start.

Bonus options (Samurai Shodown 1):
Use any character to successfully complete story mode and save the game. Enter versus mode, highlight the "Option" selection on the main menu, then hold L1 + L2 + R1 + R2 and press Start.

Fight as Kuroko or Mizuki (Samurai Shodown 2):
Select versus mode. Press L1 + L2 + R1 + R2 at the character selection screen. Move to the right to play as Mizuki or to the left to play as Kuroko.