In the downloadable content The World of Recycled Vessel (15 Nightmares), two sets of costues can be unlocked by completing doors 1 and 2. You can activate the costumes in the options menu in the title screen of the game.

Both costume sets alter the appearance of NierKainéEmil during gameplay and non-cgi cutscenes. They'll also replace Yonah during the loading screen with Kainé and Emil respectively. Further, both change the skin of magic orbs, shot by enemies, to Emil's face.