There should be no side characters in your novels.
In real life, there's no such thing as a side character. Everyone is the main character of their own story. Everyone is interesting in their own way. Everyone has something to share, learn and grow from. Everyone has battles. No ones sole purpose is to help someone else's storyline.
It should be the same in books.
Characters should never do something based on plot reasons alone. They need their own motivations and reasons for doing everything. It doesn't matter that the reader knows it, it only matters if you do. They should have backstories and an arc, basically, they should be eligible for their very own companion novel.
Because people will notice if your characters are only tools, if they're only there for the MC. And it will make all your characterization seem flat.
Are your side characters fleshed out? Can you think of any examples where every character had their own story?