Go to the Barracks and select "View Soldiers". Select a soldier, then select "Customize". Note: This can also be done from the "Customize" menu at the "Deploy" screen. Change the name of the soldier to one of the following entries to unlock the corresponding character. Note: This will prevent achievements or trophies from being earned.

Hero	Name
Joe Kelly (Heavy)	Joe Kelly.
Ken Levine (Sniper)	Ken Levine.
Otto Zander (Assault)	Otto Zander.
Sid Meier (Support)	Sid Meier.