Updated: Jul 5, 2022
Be intentional with the partnerships you make in your stories.
I really got to know my first born child once I had my second (I know, I know. You're not supposed to compare your children. Whatever.) Putting a mirror up against your protagonist helps us get to know them better.
The Odd Couple: Oscar's slobbery showed us Felix's neat-freakness, and vice versa. Yes, this is the most obvious example. I figured I would start slow.
Forrest Gump: Jenny's risk-taking showed us how cautious Forrest was.
Juno: Paulie Bleeker's nervousness highlighted Juno's fearless straightforwardness.
Zootopia: Nick Wilde's cynicism challenged Judy Hopps' optimism and put it in full view.
CODA: Ruby's lack of confidence in her singing was made more apparent by how unafraid her partner, Miles, was about singing in public.
You don't have to make everyone an opposite to your protagonist, but your secondary characters should balance out your protagonist like your potatoes balance out your steak (or your broccoli and your tofu, for you veggies out there.) You don't have to make them a good couple that wants to get married in the end (they can want to kill each other the whole time!) but do use your secondary character's differences as a mirror to illuminate your protagonist's assets and liabilities.