Unlock characters:

Duckling        Beat the game with Tom or Jerry.
Butch           Beat the game with Duckling.
Spike Jr.       Beat the game with Butch.
Tuffy           Beat the game with Spike.
Spike           Beat the game with Tuffy. 

Unlock Teamplay mode:
     Beat the game with Spike.

Cheat mode:
     Beat the game with all characters under any difficulty setting.