Who is Miss Lupescu from The Graveyard Book and what is their importance?

Miss Lupescu is similar to a werewolf, a Hound of God who can turn into a grey dog when needed. She arrives to care for Bod when Silas has to go away on one of his many unexplained absences. Bod initially dislikes Miss Lupescu, but when she saves him from the ghouls, he comes to understand that she isn't cold and uncaring, but rather, concerned about him and his well being. In the end, Miss Lupescu makes the ultimate sacrifice by giving her life to protect Bod from the Jacks of All Trades.