Any time you purchase content such as heroes, mounts, emotes, etc., you can find them under the "Customize" tab in the top navigation.
From here, you can set all of your favorites for each that you can access and switch to anytime you are in-game.
Also from the Customize menu, you can change the head, torso, legs, and even the backpack of your character! Do this by clicking "Edit" on your selected character.
Want to throw Karl's head on Advanced Trooper's body? How about Crystal's upper gear on Sonia?
Can't figure out what to swap? Hit the "Random" button in the bottom-left corner to randomize the items in the slots from the content you own. Additionally, any clothing item included with the characters you have purchased in the Shop will be available for any of your characters. You can even tint the clothing items to change up the colors as well - the combinations are nearly endless!