DeadWing

Yellow Jacket (Junkyard Level)-shoot the plane that circles the arena. If you hit it with Flame or Homing missiles, it will start on fire. It will descend and crash into the building in the quarry. Go to the wreckage and drive into the hole in the building. You should eventually come to a sewer with a pole/pillar in the middle. Shoot the control panel on this pole. You should get a message saying that you have unlocked Yellow Jacket.

Warthog (Suburbs Level)-go to the carnival where the Ferris Wheel is. Turn around and you should see a bank with a gas can power up on it. (It is to the right of the hill leading back down to town). Aime yourself at the power-up and jump of the bank. You should land on top of one of the factories. Shoot the raised section of the roof and drop inside the formed hole. Shoot the control panel inside and a message should appear saying you have unlocked Warthog.

Minion-you must complete story mode with all of the characters. (original and unlockable) You will get a message saying 'Beware of Minion'. You can now play as him.