Beat the Battle Nexus Tournament in Tournament Mode.
Karai
Beat the Foot Fight in Tournament Mode.
Splinter
Beat the game and you can use Splinter in place of Donatello
Casey Jones
Beat the Open Brawl Tournament and Casey Jones will be playable in place of Raphael
3/15/2005
Unlock original TMNT arcade game.
When on the rooftops in stage 9-1, start smashing the many crates to find an artifact. Take that artifact back home, and give it to April. Once she appraises it, she'll tell you that it is the original TMNT aracde game. Now go to the title screen, and go under options. The original TMNT arcade game will be there.
2/28/2006
Unlock Casey Jones, Karai, Splinter and Slashuur
There is another way to unblock these four characters. Beating the original TMNT Arcade Game also we will obtain to Casey Jones, Karai, Splinter and Slashuur.