Side characters are often beloved, both by their creators and the audience. While having a great ensemble cast is important in any novel, there becomes an issue when those characters begin to steal the show. While it can’t be helped for everyone, when your side characters are consistently being found to be more interesting and compelling than your main characters, you need to address the idea that perhaps you’re telling the wrong person’s story. If your side characters are too interesting and your mains too bland, your audience will often find itself caring much less for you protagonists and more for those characters who only show up every two-hundred pages.
This comes with a warning, though. Some of the interest that comes with a side character is the mystery and power they hold. That part of their intrigue disappears if you decide to delve too deeply into their stories. Some side characters simply need to remain side characters, and their stories are actually quite boring. If you find this to be the case, but your character is still outshining your protagonist in every scene, you have one of two options: make your side character less compelling or make your main character more so.
Remember: your main character is your main character for a reason. While they don’t need to constantly be the star of the show, they also shouldn’t be consistently ignored in favor of less meaningful characters.