Several years ago I attended a writing workshop at LDStorymakers that was focused entirely on creating side characters. One of the points made that struck me most was that when you create strong secondary characters, you make your novel feel authentic. You make it feel real.
This is because as an audience we don't feel as if all the side characters exist for the sake of the main character or the plot. They feel like real people with lives that extend beyond our protagonist. And yet sloppy side characters aren't uncommon. You've probably seen them before--the love interest that is only there to kiss the protagonist, the mentor that's only there to give the main character special skills, that poor geeky kid who's only there so that the main character can show off how kind and caring he is by sticking up for the weirdo, and of course, how can we forget the two-dimensional bully that every hero has these days?
Keep in mind that none of these character roles are bad or wrong per se. In Harry Potter, J.K. Rowling gave Harry a bully, a mentor, and a bunch of geeky kids he stands up for, but Rowling is a master at creating real, authentic secondary characters, to the point that it's not uncommon for fans to point to one as their favorite (Luna Lovegood, Fred and George Weasley, Dobby, McGonagall).
The trick is to make your side character feels real, and in this post, I'll give you some tips on how to do that.