I didn't want her in my story. When I was writing Joy Returns!, Margaret showed up in my head and refused to leave. She was loud and opinionated. She had a fondness for floral dresses and big hats. She informed me that she was Samantha's grandmother. When I asked if she was her mother's mother, she proceeded to give me an earful. According to Margaret, Betty was a beguiler. She had even, Margaret whispered to me, caused her husband's death.
Well, I knew that wasn't true. Samantha's father had died in a car accident. Betty had not even been on the scene. It had happened because he had wanted to drive through the night to return home, but could you blame Betty for that? She hadn't even known. It was meant to be a surprise. I was aware of all these things, but I didn't mention them to Margaret. I could already tell that it would be a waste of my time.
I did ask her to go. She shook her head in exasperation, stomped her foot, and kept talking. She informed me that she expected to be a key player throughout the book, and she demanded that she be introduced in the first chapter. I said no.
Then I wrote a draft of the first chapter with her in it. I just wanted to prove to her that it wouldn't work. When it was done, I rewrote it again without her in it. Then I wrote it with her in it again and then without and then in, then out, until finally I gave up and surrendered to her demands.
Over time, Margaret became one of my favorite characters. I was quite fond of her dramatic entrances. Once she trusted me, she let me know her vulnerabilities. There were times in the story when I cheered her on.
So, if a character ever shows up in your head, my advice to you would be to make room for her. Give yourself time for a good chat. Ask her what she wants. Listen. Obey. It's really the smart thing to do.