Who will your characters turn out to be?

When I begin writing, I have an idea of who my character is going to be. But you can't predict how a child is going to grow up and sometimes my characters surprise me who they become. I started writing one of my characters thinking she'd be nice and help my main character. It appears she's taken on a life of her own and has other plans. As I continue to write this character in the story, the more she hinders my main character.

So far it's not a bad change, just not what I was expecting. I could go back and try to make her be the person I want her to be, but I think it's better if I let my characters grow into their own. Have you ever had this happen to you?