Outfits in Final Fantasy XV are automatically unlocked over the course of the story. There are only six of them and they can boost some of your stats (increase your health, boost damage or defense).
You can equip them in the “gear” section where you also choose your weapons. The two Kingly Raiment Outfits are a reward for beating the story. Everything else is available from the beginning.
Here are all Outfits (click images to enlarge):
|Price’s Fatigues (No Jacket):|
|Casual Outfit (No Jacket):|
|Kingly Raiment (No Jacket):|