Fight as Kemo Claw:
Highlight the "Arcade Mode" option, then press Left(3), Right(3), Left, Right, Left, Right.

Fight as Nat Daddy:
Unlock Kemo Claw, highlight the "Arcade Mode" option, and press Right(3), Left(3), Right, Left, Right, Left.

Fight as Damien Black:
Unlock Kemo Claw and Damien Black. Highlight the "Arcade Mode" option and press Right, Left, Right(2), Left(2), Right(3), Left(3).