Was just talking with @Pixileanin about this point and figured I'd share for one of our Camp FFT prompts. 

Sidekicks matter folks. That's some of the best writing advice I was given. A reader gets to know your lead character - even if you're writing from their POV - you get to know them from how they react to the world and how the reactions of people around them compare. It's the same for world-building too. Side characters are what give your story perspective and momentum. They're your temperature checks. 

Sure you can write a ton of inner monologues and diatribes about whatever scenes your lead is bouncing between - but how much better is it for them to express those thoughts in a conversation with someone else. To see the reactions to their thoughts and feelings and get feedback on the choices their making - for good or for bad. Give them a scene partner!

Give your lead a best friend they can get advice from or a sibling that will tease them about their crush or even a consistent rival that they're always trying to outsmart or one-up to show their competitive side. How does your character see the world different from them? Where do they see things the same? How does your character react to someone they trust telling them they're acting out of character? How do we even know if they're out of character unless someone else pops in to notice it?

Everyone should have a purpose in your story. How do these minor folks bring more to the table? How do they heighten the stakes? How do they bring out more or different emotions? Or maybe they bring a new take on the world as a whole to the table. If they're important enough to give them a name then give them a perspective to! 

Sorrin is one of my favorite side-characters in my current WIPs. He's the youngest of the gang, barely eighteen at this point in the story, from the poorest parts of the kingdom, has an incredible amount of talent so he doesn't have to work hard to be great at things.
- He's young because I wanted someone for Wit to be protective of, for the reader to want someone to be protective of them.
- He's poor to show how things work in the kingdom. Rank and nobility can go out the window when runa is involved.
- He's a flirt and rascal to contrast with Koen who's got plenty of charm, but with a more focused eye. 
- He's traveled with Koen and Jessa for long enough to get them and be someone for Lena to talk to about them.
- He's got a talent for runa without really trying to show off that side of the magic system and how runa works.
- He's cheeky and inattentive because I think it's adorable and funny. This is just for me