WinBack: Covert Operations

All multi-player characters:
Press Up, Down(2), Right(3), Left(4), then hold C-Up and press Start at the "Press Start" menu.

All the Bosses as well as the engineer and a few army characters will be unlocked in multi-player mode.
Most of the Bosses have odd guns and may not be able to back up against walls or roll. (Information from Cheat Machine)