Players, rather than choosing from one male or one female, can simply pick gender and customize aspects from hairstyle, hair color, eyes, facial features, etc. and give them a variety of clothing. You can change them from your room, and while most of the clothing is unlocked, you can unlock the clothing of past characters of the same gender (such as Red, Ethan, and other clothing for the male character.) after beating the Elite Four.
Accessories are a brand new element for Sunlight and Midnight. At the Fashion House, you can buy and equip Accessories such as scarves, hats, and other items for any of the 746 Pokémon. Each accessory comes in a variety of colors.