If your brother is depicted as a couch potato in your autobiographical novel, it means that you described him in a certain way, so that he came across as a lazy TV watcher.
The adjective depicted means "shown or represented." A person or thing can be depicted in a particular way through a description, a drawing, a photograph, or any other artistic medium. Whenever a vivid picture of something — like life in Victorian England — is presented, that thing is being depicted. The Latin root depictus comes from the prefix de, meaning "down," and pingere, "to paint," and it means "to portray, paint, sketch or imagine."