At the title screen take the second controller and press UP,X,L,Y,DOWN,B,R,
A,UP,X. Then select VS. mode, press left on controller 1. Now you can play
as any boss.

Some special moves:
Kari     away+UP+X+A                     Punch-a-thon
         A+X (when Sepcial Power full)   Lightring scream
Ratking  A+towards                       Fire kick
         DOWN+B                          Flying Wrist Thrust
         A+X (when Special Power full)   Ball of light