Players can customize their characters (or virtual avatars) in OpenArena. To do so, go to "Setup" --> "Player" from main menu, or from ESC menu if you're playing. This will bring you to the Player Settings page, where you can customize your name, "handicap" level, "effects" and your "model/skin" (character). You can also access this screen by clicking on your character's picture in the "choose level" screen in Single Player Deathmatch mode.