
Every character in a fictional story needs to play a role, whether a strong antagonist standing in the way of your hero or a waiter bringing a check. But each character must be placed purposefully and strategically, all to move a riviting plot along.
We all need a supporting cast in our novels. Secondary characters have to be in there, unless your book is about a guy stuck on a deserted island the entire time. But even in that instance, an animal or even a volleyball (sorry, had to put that in there from Cast Away) can play the role of a secondary character.
There are plenty of great movies where even the hero is an animal (The Incredible Journey is one that comes to mind) or something not human. but whether your secondary characters are human, feline, canine, or bovine, they need to be fully human in their characteristics (well, maybe cats can getting away with just saying no).