The Girl on the Dock is Petra Morganstern from Lippert's James Potter series. She is no doubt a troubled young girl, and this is her sad, dark story. I read this in preparation for the third James Potter installment, and I am glad I did. I wish now that J.K. Rowling had written side stories on each of the original Harry Potter characters!
This is a short side story, but definitely enchanting on its own or between the 2nd and 3rd books. Enjoy.