There are three alternate outfits in The Evil Within 2 – the Classic, the Drifter, and the Professional – all of which can be unlocked upon completion of the game.
Shinji Mikami is best-known for his work in the survival horror genre, having directed both the original Resident Evil game and the critically-acclaimed Resident Evil 4. For whatever reason, alternate outfits were available for the playable characters in both games, all of which were unlocked when the player reached certain milestones (usually by beating the game).
Unlockable outfits were notably absent from Shinji Mikami’s The Evil Within, however, they have made a return in its sequel, The Evil Within 2.
It’s not entirely clear how you earn each outfit, as some players have reported only unlocking one or two of the three outfits, which would imply that you unlock them over the course of your first game, and that they are only awarded to you once you beat it. Your best bet is to complete as much side content as possible – gather as many collectibles as you can, complete as many side quests as you can, play on harder difficulties, etc.
A comment on this Youtube video suggests a possible method for unlocking the Classic outfit (and you might be able to find more answers as more people comment):
You can swap Sebastian’s attire by interacting with the coat rack in his office room – however, you should be aware that you cannot change his outfit in the unlockable Classic difficulty.
Those are all the alternate outfits currently in The Evil Within 2, however, most Resident Evil games have had DLC introduce more unlockable outfits. So, if you’re an alternate outfit aficionado, it might be worth staying up-to-date with The Evil Within 2 news, as no DLC has been announced as of yet.