Miss Lupescu first enters the scene when Silas has to leave the graveyard for a while. She's the person Silas found to be Bod's guardian while he's away. At first she seems like the worst kind of substitute teacher. She gives Bod lessons, but they're all boring. She's really strict and says he must pay attention. And she makes Bod eat nasty food such as dumplings swimming in lard.

However, she was actually being helpful. Miss Lupescu wasn't only feeding Bod’s body with home-cooked, high-nutrition food, she was also teaching him discipline and practical knowledge. She told him about all the kinds of "people" in the world, including ghouls and Hounds of God. She also taught him ways to call for help in every language in the world. One language, Night-Gaunt helped Bod escape Hell and some ghouls that decided to turn him into one of them. Once Bod realized that Miss Lupescu's lessons actually are useful in his life, he became an enthusiastic student, drinking up all the knowledge she put before him. Bod and Miss Lupescu actually became friends, and she even took him to a football game. She also saved him in Hell, because she was a Hound of God, and helped him escape and come home to the graveyard.

After Bod finally realizes that she and her big grey dog are the same, she writes him a note of explanation:

Those that men call Werewolves or Lycanthropes call themselves The Hounds of God, as they claim their transformation is a gift from their creator, and they repay the gift with their tenacity, for they will pursue an evildoer to the very gates of Hell.

This shows Miss Lupescu is religious, and she believes in God. Second, she suggests that all werewolves are Hounds of God. So unlike other books, werewolves in The Graveyard Book aren't evil. 

Like Silas, Miss Lupescu is a member of the Honor Guard and she dies battling the Jacks. She seems to be an important part of the fulfillment of the prophecy that if Bod reached adulthood, all the Jacks would be destroyed. She also was an important friend, to Bod, and many others.