Earn an "A" or higher rank in the indicated Mercenaries mode mission to unlock the corresponding character.

Chris (Safari): "The Village" mission.
Chris (S.T.A.R.S.): "Experimental Facility" mission.
Jill (Battle Suit): "Ship Deck" mission.
Jill (B.S.A.A.): "Public Assembly" mission.
Sheva (Clubin): "Ancient Ruins" mission.
Sheva (Tribal): "Missile Area" mission.
Wesker (Midnight): "The Mines" mission.
Wesker (S.T.A.R.S.): "The Prison" mission.