You can unlock the characters indicated below by meeting the corresponding requirements.
Cobar - Finish Hard mode
Commando - Finish the game in the 2-player cooperative mode on any difficulty setting
Grenadier - Acquire gold medals in all challenges
Hakha - Finish the game, through Chapter 5, in cooperative mode on any difficulty setting
Luger - Finish Chapter 5 on any difficulty and earn gold medals on all of that chapter's Challenge games
Specialist - Win 10 matches in Ad-Hoc or Infrastructure multi-player
Stratson - Win 100 matches in Ad-Hoc or Infrastructure multi-player