When you get an 'A' rank in Mercenaries mode with the indicated character the corresponding bonus costume will become available.

Pirate - Leon:
Get an 'A' rank in Mercenaries mode with Leon.

Puffy Jacket - Jake:
Get an 'A' rank in Mercenaries mode with Jake.

R.P.D. - Helena:
Get an 'A' rank in Mercenaries mode with Helena.

Racer - Piers:
Get an 'A' rank in Mercenaries mode Piers.

Samurai - Chris:
Get an 'A' rank in Mercenaries mode Chris.

School Girl - Sherry:
Get an 'A' rank in Mercenaries mode Sherry.

White Dress - Ada:
Get an 'A' rank in Mercenaries mode Ada.