Right here, ViVi...
Warthog:
Enter story mode, select any character and go to the suburbs for the second level. Go to the hill with the Ferris wheel and look for the Gas Can power-up. Gain some speed and try to land in the middle of the building that is right in front of it. You should fall through the building and see something that looks like a computer panel. Shoot it and it will reveal Warthog as a playable character.
Axel:
In the Freeway level, look for the repair station. In that area, look around and you\'ll find two cranes. One holds a pipe, the other, a cage like item. The cage crane has a yellow cage like box on the left side. To get axel, destroy the yellow box. Aim for the box on the hill with a fire missile. Move around. If you don\'t make it in one place, aim for the box in another place on the hill. This is tough, aiming for the box is hard, and with cars pounding you from all sides, it\'s even tougher.
ManSlaughter:
In the Prison level, after you land on the island, look at the docking area. You would find a health pickup somewhere near a bunch of crates at the base of the ship. Destroy one of the crates to reveal a ramp. Go up the ramp and get the health. Part of the ship will be near the crates. Fire at that part of the ship. It will reveal a hole. Shoot the lever to reveal Manslaughter.
Credit: The Shrike