Does the Influence Character have to be a person?
Does the Influence Character (or any other character for that matter) have to be a person? For instance, if the story is about someone facing the desert alone.
No. No character must be a person. They may be animals, minerals, or vegetables. HOWEVER, giving an inanimate object like a desert a "point of view" or "alternative paradigm to that of the Main Character" may take some clever storytelling for your audience to understand. Do not confuse normal, every day obstacles (like cacti, snakes, heat, etc.) for the type of personal impact that the Influence Character has on the Main Character. The IC helps strip away the MC's justifications or, in some instances, helps to build the MC's justification. The IC's impact is very personalized, whether or not the IC is even aware of the MC.