Costume
From VanRO Ragnarok Online
A costume is an equipment which is placed on the costume tab of the equipment window. Costumes do not take up the actual equipment slot. For example, a Purple Fairy Wing costume which is equipped in the middle headgear position will make it appear as though you are wearing Purple Fairy Wings, meanwhile you can actually wear (and enjoy the stats from) another mid item such as Sunglasses.
Costumes are only available on RebirthRO: SE.