Skins or costumes change the appearance of your pigments in battle when you're playing and even when you're defending against someone else. Most skins you have to purchase with real money, but a few you can earn by collecting cards in battle like you would with mercenaries or medals.
PLEASE NOTE: you can only wear ONE skin at a time, so if you're going to get a skin, go for one only, this is also inconvenient for when you buy a special deal sometimes you get two skins at a cheaper price but are only able to wear one.
arm cannon uniform (mega man):
Storm toaster skin: