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.

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.