At first this title seems pretty obvious: The Little Prince is about the main character, the little prince, who goes on adventures and meets the narrator. What else could this book be called, really? None of the other characters are important enough to deserve the whole title. Even the narrator doesn’t have a name. The book exists so that the narrator can remember the prince. From that perspective, it simply has to be named after the prince. Otherwise, it wouldn’t be a very good memorial.
If you want to be fancy, you could refer to the title in its original French: Le Petit Prince. That’s not so hard to match, word for word, against the English version. “Le,” the article, is basically the same as saying “The.” “Prince” is the same word in both languages. “Petit” can mean “little,” like in the English title, or “small” or “young.”