Fight as Amakusa:
Quickly press A, Y, X, B at the Takara logo screen. Hold L + R during character selection in versus mode or countdown mode to transform into Amakusa.

Amakusa's moves:

Taunt: Heavy Slash or Kick

Force Ball: All four buttons at once.

Kamikaze Bomber: Triangle Jump, then A.

Fire Dragon: Quarter Circle Back, Slash.

Teleport: Quarter Circle Forward, then Slash to go forward or Kick to go back.