Who is Liza Hempstock from The Graveyard Book and what is their importance?

Liza Hempstock, who'd lived on London hundreds of years before is a witch. Murdered at the hands of those she frightened, Liza cursed her murderers before her death.... the first time she'd ever cursed anyone. Buried in unconsecrated ground allows Liza more freedom than the others in the cemetery, and she uses this freedom to assist Bod when he needs her.