Complete the game once on any difficulty to gain the Special Key to open the costume room where you can change you clothes after completing the game the required amount of times.
Jill's Sarah Connor costume: Complete the game once with Jill on any difficulty.
Jill's Resident Evil 3 costume: Clear the game twice with Jill on any difficulty.
Chris' The Mexican costume: Clear the game once with Chris on any difficulty to unlock his costume where he dresses up like Brad Pitt from the movie The Mexican. Rebecca also changes her costume to look like Julia Roberts from The Mexican.
Chris' CODE Veronica costume: Clear the game twice with Chris on any difficulty.