Like a Dragon Gaiden: The Man Who Erased His Name

For this achievement, you must buy outfit items that your character will show on-screen in regular adventure as well as items exclusive for Coliseum battles. They will be available at the Castle Boutique shop in the Castle Complex, and you'll be introduced to the shop itself in the main story in Chapter 2, where you'll need to put an outfit for both open adventure and Coliseum to progress. When you enter the shop for the first time, interact with the clerk that has the bubble icon of a dress shirt and tie with a watch next to it above his head, and you'll be taken to the outfit menu screen, where you'll have 2 categories of outfits with 5 open slots each, one for casual wear and the other for your Coliseum fights. Pick an open slot, and then go through the item categories covering the Body, Head, and Other sections, buying out each accessory with what you can afford. The achievement should pop immediately after purchasing the item that counts to the 15 items you now own.