Leveling up with xp in this game also gives you Armory Keys and Access cards every once in a while. To unlock a costume, head into Bonus Features from the main menu, choose Archives and
over to Costumes. There are 24 costumes to unlock and you need 1 Armory Key to unlock a costume. Simply choose which one you want to unlock (I personally chose Bats) and hit
to confirm. Bleep bloop, achievement unlocked.