Fight as Grant
Highlight Rock at the character selection screen and press Left to highlight Grant. Press any button to select him.
Fight as Kain
Highlight Terry at the character selection screen and press Right to highlight Kain. Press any button to select him.
Beat your opponent until he goes down. Quickly press Left + Start, Right + Start, or Right + Down before the "K.O." message appears.
Hold Start at the character selection screen. It is possible to fight as Kain or Grant randomly through this method.
Select victory pose
Press any attack button immediately after defeating your opponent.