Unlock Purple Gremlin (Race Mode)
When you start the Thugsville board, you will noticed two elevated train tracks on either side of the map. Follow the right track to a tunnel. Inside, take the left fork and you will find the unlockable. He is only usable in race mode.

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

Prof. Gutenburg:
At torpedo run there will be three subs on your radar. One of them is a yellow sub with Prof. Gutenburg on the front. Shoot two torpedoes at it and you will unlock Prof. Gutenburg

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