Go into adventure mode and go to the torpedo run. Three subs will display on your radar. One of them is a yellow sub with the professor on it. Unlock the code by shooting two torpedoes at the yellow sub.
Island Jack
At danger island when you get to the jungle go below the trees. You will see a giant snake wrapped around a tree branch near the end on the left side of the jungle, shoot it and you will unlock Island Jack.
Andre LaToilette
In Bigfoot Mt. at the beginningof the level you will see a house on a hill. In front of that house is a snowman. Shoot the snowman and eventually it will say you have unlocked Andre LaToilette.
Sheik Abdoul
In the Cave of Blunders you start above a Arabian city. In the center of town there is a small area with pillars where Sheik is being bullied by a palace guard. Kill the guard to get Sheik Abdoul.
Cactus Rose
In the beginning of Coyote Canyon on your right will be a town. In that town there will be a girl being hanged. Shoot her and it will say you've unlocked Cactus Rose.
Sammy Wasabe
Go to monster isle. Shoot a ninja by the shrine in the middle of the jungle. If you did it right you will hear "you unlocked Sammy Wasabe".
Pilot X
On the last level in Adventure mode, X, beat Pilot X by destroying his robot. Pilot X will become a playable character in all modes except Adventure Mode.
