Jill begins the novel not as a companion but as a mentor to Mike. Like Jubal, she teaches Mike how to function in human society and culture. But as the novel progresses, she becomes more of a disciple of Mike's since Mike teaches her the Martian language. By teaching her the language, he also teaches her how to view the world from a Martian perspective. When Mike leaves Jubal's to travel the world, he chooses Jill as his companion. Doesn't get more clear-cut than that.