At the character selection screen, highlight Rock and press Left to highlight Grant. Press any button to select him.
At the character selection screen, highlight Terry and press Right to highlight Kain. Press any button to select him.
At the character selection screen, hold Start. It is possible to fight as Kain or Grant randomly through this method.
Beat your opponent until he goes down, then quickly press Left + Start, Right + Start, or Right + Down before the "K.O." message appears.
Immediately after defeating your opponent, press any attack button.
Successfully complete Survival mode to unlock artwork, including the ending artwork for the character used to complete the game.