There are a total of four different costumes to unlock in the game
The first two are unlocked for simply beating the main storyline. This will earn you:
•Leon’s RPD uniform &
•Ashley’s ‘Pop-Star’ outfit
Beating the storyline will also unlock the mini-game ‘Separate Ways’ (a mini-story involving Ada.) Beating this will earn you:
•Leon’s Gangster outfit &
•Ashley’s suit of armour
The achievement should pop after the ‘Separate Ways’ credits on returning to the title screen.