I'm in the camp that says all characters should relate in some way to the main theme of the story. It's what they're there for. Each character represents a particular take on the theme, and they square off in conflict over their stances toward it. If the theme has to do with friendship for instance, you should have groups of people that demonstrate different takes on it. Some are good friends in different ways, some are bad ones, and that plays itself out over the course of the story. This way every interaction has something to do with the main theme.
I’ve often heard that all characters should have a goal, even side characters. I would also suggest that side characters goals should at least partially interfere with the Hero’s goal. Example: Let’s say the Hero wants to get the girl and find true love. And the side character is his womanizing cousin who gives him all sorts of tips on how to be a player, because they grew up together and he wants his cousin to be successful getting laid. And maybe the hero naively follows the advice, which helps him get some numbers at a club, but turns off the girl he is really interested in. See, the agendas aren’t 100% aligned. Cousin is inserting his agenda in place of the hero’s, and even though he thinks he is helping and is not acting as a traditional antagonist, that misalignment throws the hero off.
That's a really great answer! I agree that the side characters are always going to have their own agenda and will have the outcome be different from what the main character expects
I wrote a story where the side character or protagonist took over and lead me down a path I didn't want to go. It was about a young girl sent to a tough love camp and turned into a cartel, black ops, convoluted mess and at 22000 words; I stopped. It's not my fault, they should have listened to me.
Purpose. There are a lot of side characters who are there because the author liked their personality, but they don't offer much in terms of the story. And purpose can be something as simple as being the cashier at the register when your character is making a mundane purchase to being the comic relief. Or serving as the mentor. Or are they a best friend to the main character? Often, side characters can be broken into their archetypes and those archetypes can provide a good framework for their purpose.
