The usual convention is to start your story with the main character. That way,  your reader immediately knows who the story is about. Prologues are the only exception. If something occurs before the main story begins that involves another person, often a prologue functions well to present that information.

Sometimes another character comes along who is so strong, that he or she takes over the story. It’s okay to have several strong characters in a story, but who does it really belong to? This is not always easily answered. Sometimes two characters come together who have separate story arcs. I don’t have all the answers to that situation and am dealing with just that in my current WIP.

However, if someone else clearly takes over, leaving your original main character/protagonist in the dust, then it’s time to reconsider who your protagonist really is. If it changes dramatically, you may have to go back to Chapter One and introduce that person first.

Main characters in complex stories with well-developed plots are not always that easy to identify. How often has your favorite character in a book been someone other than the protagonist? Side and back stories are always an option with these folks as well.