When characters are boiled down to one identity or personality trait, they stop being people. When portraying asexuality, it’s especially important to show fully fleshed out characters, because of the history of bad representation. This is harder in some mediums than others. For example, in visual art it’s often necessary to use simple, visible indicators to portray a subject. At the end of the day, making sure that asexual characters are defined by their personality, not their asexuality, is a good way to start.