Fight as Geese:
Highlight the sound test selection on the options screen and press Left + A to unlock a "Soft Dip" option. Use this new option and set its value to 11111110. Then, return to the character selection screen, highlight Billy and press Start. Alternatively, complete the game without using any continues, highlight Billy on the character selection screen, and press Start.

Fight as Iori:
Highlight the sound test selection on the options screen and press Left + A to unlock a "Soft Dip" option. Use this new option and set its value to 11111110. Then, return to the character selection screen, highlight Krauser and press Start. Alternatively, complete the game without using any continues, highlight Krauser on the character selection screen, and press Start.