Who does the gray ghost symbolize in chapter 31 of "To Kill a Mockingbird"?

In order to understand the symbolic meaning of the book The Gray Ghost, you need some background information. 

In the beginning of the book, Jem, Scout, and Dill are preoccupied with Boo. They want him to come out. Jem and Scout warn Dill that this was a dangerous thing. However, Dill persisted. On one occasion, Dill dared Jem to touch the front door of the Radley house. Dill said that if Jem would do such a thing, he would give him his copy of The Gray Ghost.

Scout says that Dill never turned down a dare and so he did it. The Gray Ghost represents Boo or better yet what the children imagined Boo to be. 

At the end of the book, Atticus see the copy of the book, and Scout asks him to read her the book during bedtime. Atticus refuses at first, as the book is scary, but Scout insists. She says that she is not sacred. In fact, as Atticus starts, Scout fall asleep immediately. 

If the gray ghost represents Boo, then Scout really has nothing to fear, as Boo and Scout are now "friends." What seems fearful no longer is. The ghost might still be gray, but the ghost is friendly.