There are two different versions of Jill to unlock for the mercenaries mode in Resident Evil 5.
The first is Battle Suit Jill, which is unlockable by getting an A rank on Ship Deck in mercenaries mode.
The other outfit, BSAA Jill, is unlocked by getting an A rank on Public Assembly in mercenaries mode.
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.