Enter the following cheats at the title screen.
Unlock All Characters: Hold WHITE + BLACK and enter UP, RIGHT, LEFT, DOWN, release WHITE + BLACK and enter L, R
Unlock B-Side Audio: Hold L + R and enter DOWN, X, X, UP, X, X, LEFT, X, X, RIGHT, X, X
Unlock "Through the Portal" bonus Rikimaru mission: WHITE, UP, BLACK, DOWN, L, R, LEFT