There are two extra versions of Chris to unlock for the mercenaries mode.
The first is Safari Chris, which is unlockable by getting an A rank on The Village in mercenaries mode.
The other outfit, STARS Chris, is unlocked by getting an A rank on Experimental Facility.
Getting an A rank requires either 40,000+ points in single player mode, or 80,000+ in either co-op mode.
If you’re looking for some help for the mercenaries mode, check out my mercenaries mode strategy guide.