I agonized over the last couple of weeks whether or not to delete one of my characters, my mc's great grandmother. A different one I just chopped, easy peasy, cut her out of the story and done, we shall call her Shaye (okay that was her name) was part of a subplot that I was wishy wash about. I felt relieved when I cut her.
With Grammy I thought well, is she really accomplishing anything? Surely I could combine her purpose into a different character. I couldn't really put my finger on why she was integral to the story.
So I tried to delete her. But nothing flowed. I rewrote the opening a couple times and I just didn't like it.
And then I had an ah-ha moment. I realized, that yes someone else could fulfill her purpose, but Grams set the tone. She was cryptic, she was wrinkly and there was just something about her I couldn't recreate in my other characters, something a thirteen-year-old boy couldn't do. So I fleshed her out and made her matter more. And I like the opening a lot better, isn't that great when that happens?
So moral of the story: all your characters need a purpose, and if you can't put you finger on it try to delete them, see what happens.