Enter these codes at the title screen:

Yami Yugi:
R, LEFT, B, RIGHT, Up and R, RIGHT, DOWN, A

Kaibaman:
DOWN, B, LEFT, RIGHT, RIGHT, R, Up and R, A

Yami Bakura:
L, DOWN, RIGHT, LEFT and B, R, DOWN, DOWN, A

Grandpa:
L, RIGHT and B, RIGHT, B, R, LEFT, DOWN, A