Unlock Island Jack
In adventure mode go to danger island. when you get to the jungle go below the trees. when you see a giant snake wrapped around a tree branch near the end on the left side of the jungle shoot it and you will get island jack
Unlock prof. gutenburg
Go to adventure mode and go to torpedo run there will be three subs on yor 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
UNLOCK SHIEK ABDUL
IN THE TOWN AREA IN THE CAVE OF BLUNDERS LEVEL, LOOK DOWN IN A LITTLE ARENA LIKE PLACE AND YOU'LL SEE A GUARD TRYING TO ATTACK A TOWNSPERSON. SHOOT THE GUARD AND YOU'LL UNLOCK SHIEK ABDUL.
UNLOCK THE MARCELS MORESO BRO.S
IN THE BOMBSBURG LEVEL, BEFORE YOU REACH THE AREA WHERE THE MACHINES ARE, LOOK IN THE LAST FEW BUILDINGS ON THE LEFT AND YOU WILL FIND THE MARCELS MORESO BROTHERS. RUN INTO THEM TO UNLOCK THEM.
UNLOCK CACTUS ROSE
IN THE COYOTE CANYON LEVEL, LOOK IN THE OLD WESTERN TOWN AT THE BEGINNING OF THE RACE. THERE IS A CARRIGE ON THE RIGHT SIDE OF THE TOWN THE CARRIGE WHERE YOU COMPLETE THE DAMSEL IN DISTRESS SUBGOAL. SHOOT THE CARRIGE AND YOU'LL UNLOCK CACTUS ROSE.
In Bigfoot Mt., when you start in the beginningof the level you will see a house on a hill. Infront of that house is a snowman, shoot the the snowman and if you shoot enough it will say you have unlocked Andre LaToilette.
Get Sheik Abdoul the Arabian man with the rocket powered flying carpet
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.
Unlock Sammy Wasabe
Go to monster isle. Shoot a ninja by the shrine in the middle of the jungle. If you did it right it the guy will say "you unlocked sammy wasabe.