Wanda, the sidekick who nearly stole the show

When I was told several times that my main character's sidekick was stealing the show, I decided to make her the main character of the next story.  In the first one (The Girl on the Mountain) Wanda is 13, abused and homeless, but resilient, wise, and strong-willed.  In that story she's a contrast to the older, somewhat naive main character (May Rose), who gives her a home and is inspired by her.

I like starting a new project with a character I know so well.  But almost immediately I'm confronted by the problem of maturity and change.  The new story takes place fifteen years later, and rough little Wanda has become a rough grownup. 

As an adult, Wanda can continue to be independent and outspoken.  Those traits can help as well as hinder the accomplishment of her goals in the new story.  But she can't retain the reactions of a child, or she will be neither loved nor a good main character.  I'm also wondering if her ungrammatical speech will make her less acceptable as a grown-up main character.  People do tend to associate ungrammatical speech with ignorance--not an accurate association, but true of our prejudices.

So help me out.  What do you know of "rough" main characters, especially female ones?  Main characters with poor speech?  What makes them lovable?