Whether it’s to move the plot forward or provide a unique perspective on an event or another protagonist, every character should have a purpose. Not just, but no two characters should have the exact same function in the story (aka cloning.)
It seems obvious enough, but while working on my space opera WIP, I ran afoul of this critical idea, and rectifying it was not a happy experience. Over halfway through the novel, I wanted to inject some new blood into the story. Enter Tara, an ex-U.S. Special Forces operative but geek at heart. She was funny, animated, and a breath of fresh air.
Five and a half thousand words later, I realized she was a younger clone of another character, Mrs. E. But, but, but! Tara was an ancient astronaut geek, so she could provide insight for the crew! So could Mrs. E. There were some great moments in those five thousand words! Too bad. Why can’t I keep her? Because when I got right down to it, she served no real purpose. I had to contrive ways to involve her in the rest of the plot, and that inevitably marginalized more integral characters.
I've no words of wisdom to impart, just the foibles from which I learn on my writerly journey. I learned Tara existed because I was tired of writing all the other characters, and I just wanted to write something new. Don’t do that, even if it feels good—and fun and exhilarating. Make sure each character compliments others instead of replacing them. Make sure they fit the plot instead of the other way around. Make sure each has a developmental trajectory over the course of the story, a lifespan longer than one scene.
Anything less warrants no more than a few paragraphs, let alone five thousand words. Failing that? Cut ‘em without mercy…and paste them into a new document for another story. ;)