There is a hidden character that can be unlocked for each class by doing specific in game tasks. They are unlocked as follows.

Character	How to unlock
Audrey (Lancer)	Earn 10 or more medals
Emile (Sniper)	Unlock Oscar's hidden potential, the let him die in battle
Knute (Engineer)	Enter the Command Room with 1,000,000 DCT
Musaad (Scout)	Beat the game