Earn an "A" or higher rank in Mercenaries mode to unlock the following costumes.

Chris's Safari costume
Chris' S.T.A.R.S. costume
Sheva's Clubbin costume
Sheva's Tribal costume
Wesker's Midnight costume
Wesker's S.T.A.R.S. costume.
Jill's B.S.A.A. costume.
Jill's Possessed costume.