Once you have your idea for a novel, you need a person through whose eyes the reader will see the story unfold. There can be more than one main character and each one must tie into the idea in some way. In romances, there are usually two -- the heroine and the hero. What a writer needs to decide is how each fits into the idea. In romances, usually the idea is love winning out. In a mystery there are often the hero/heroine and the villain. Each fits a facet of the idea. Crime does or does not pay.
I sometimes write stories with many main characters. The idea here is that each must fit the idea. In the YA I'm working on, the fourth of the story the main idea is good versus evil and this often fits into the fantasy novel as an idea but I have given my young protagonists affinities with the elements -- Earth, Air, Water and Fire. So here there are a number of ideas that must be blended into the major idea. Hard work but slowly I'm getting there. Because there are a number of protagonists on the hero/heroine side, there are a number of villains. In this cast only one has a viewpoint and that is in the final book. He has been designed to fit into the idea of the book which started with my imagining what if someone could control Earth, Air, Fire and Water.