Get an "A" rank or better in Mercenaries mode with the indicated character to unlock the corresponding costume.

Ada (White Dress): Get an "A" rank or better with Ada.
Chris (Samurai): Get an "A" rank or better with Chris.
Helena (R.P.D.): Get an "A" rank or better with Helena.
Jake (Puffy Jacket): Get an "A" rank or better with Jake.
Leon (Pirate): Get an "A" rank or better with Leon.
Piers (Racer): Get an "A" rank or better with Piers.
Sherry (School Girl): Get an "A" rank or better with Sherry.