Fight as Assault:
Press Right, Up(2), Right, Down, Right, Up(2) at the character select screen.

Fight as Mayhem:
Press Left, Right, Down, Left, Up, Left, Down, Left, Down at the character select screen.

Fight as Vitriol:
Press Right(3), Up(2), Down, Left(2), Down(2) at the character select screen.

Fight as Anil 8:
Press Up, Right, Down, Right(2), Up, Left, Up at the character select screen.

Fight as Supervisor:
Press Down, Right, Down, Up, Right, Down, Left, Down, Up, Right ,Up at the character select screen.