Twisted Metal: Black

There are a total of five characters you can unlock. They are Yellow Jacket, Axel, Manslaughter, Warthog and Minion.

To unlock Yellow Jacket, you need to play the first stage. Flying around the outside of the stage is a plane which, with proper aim or a random volley of fire missiles, you can shoot down. It will crash into the lower area of the stage and allow you access to a new area. You can unlock Yellow Jacket in there.

To unlock Axel, you need to go to the freeway stage. There's a construction zone with a couple of cranes in it. One of the cranes is attached to a crate which you need to shoot down. You unlock Axel inside that crate.

To unlock Manslaughter, go to the ship stage. Continue playing until the ship docks, and go down towards the bottom area of the ship. There's some crates down here you can shoot that will break, forming a ramp. On the ramp, shoot the side of the boat and a secret area will open up. Manslaughter is in there.

To unlock Warthog, go to the suburbs stage. Go to the ferris wheel and face the other side of the carnival area. There's a hill you can fly off of and on the other side a building you need to land on. You can blow up something on the roof to create a hole, which contains Warthog.

To unlock Minion, you just need to finish the game with every character, including the four above.