Originally, costumes were meant to be complete vanity gear that were only for display. But soon after the Costume system was implemented, Gravity began adding costume gear that had effects, allowing players to essentially wear two sets of equipment instead of one equipment set and a costume set.
Costumes are special gear items that can only be equipped onto the Costume tab of a character's equipment window. Costumes override the display of equipped gear, allowing players to customize the look of their characters without sacrificing good effects from otherwise ugly gear.
Costumes can be obtained from various sources in the game.