Victor Vran

You need 6 outfits to get this achievement. You pick one after the tutorial, and get 2 or so from leveling up. So you still need a few more. Chests and monsters can drop them, although it seems to be a pretty rare occurrence. I'm up to 7, almost done with the challenges/secrets in the main game, so I assume you'd have this by the time you finish everything else, especially if you're doing the DLC too.