Enter a soldier's customization menu, go to the character info menu, and enter one of the following names to unlock the corresponding character. Note: This only works in the single player campaign and disables trophies from being earned and removes all your previous soldiers' attributes.

Hero	Name
Beaglerush (Grenadier)	Beaglerush (nickname)
Peter Van Hoorn (Assault Build Ranger Colonel)	Peter Van Hoorn
Sid Meier (Psi Operative Magnus)	Sid Meier