Hidden characters:
Enter the following codes on the game selection screen: 

Butch
Hold L and press X, R, A, X, R(2). 
Peelgood
Hold Down/Left, then press B, Y(2), A, Y. 
Sarge
Hold X, then press L(2), Up, Down, Left, Down. 
Jack
Hold Up, and press A(2), R(2), Y, A. 
Ice
Hold B, and press Up, L(3), Right. 
Thunder
Hold Up/Left, and press Y, B, X, B(2), X, A. 
Slyck
Hold Y, and press L(2), Up, L, Left, R. 
Spike
Hold R, and press X, B(2), A, Y, Left, A. 
Enable all hidden characters:
Hold L + X, and press Up(2), Select, Up(2), Right. 

Top speed:
Hold Y, and press L(2), R, Down, Left, R at the game start screen. The
speed may now be set to ten in the options screen. 

Random character:
Press R + L at the character selection screen.