She is described as being a tall and beautiful woman with a compelling voice, however, she wears a glamour to disguise the fact that she is actually an old and decrepit witch.
In The Novels
Before the coming of the Wizard, La Mombey was an adviser to Pastorius, the Ozma Regent. After the Wizard overthrew Pastorius, he gave the regent's baby daughter, Ozma Tippetarius, to La Mombey to keep and hide away forever. The witch transformed the princess into a kitchen boy she would call "Tip" and use as her personal servant.
Mombey is introduced in Out of Oz as the new Eminence of Munchkinland. She puts Dorothy Gale on trial for the death of Nessarose Thropp, as a distraction for her plan to attack the Emerald City. She also tries to forcefully recruit Animals into the Munchkinland army by casting a spell on the bridges in Bright Lettins, which prevents the Animals from leaving, Brrr however manages to push through it when rescuing Dorothy. She transforms Liir into an elephant so as to force him to translate the Grimmerie for her, and reveals she was the sorceress who placed the spell on Princess Nastoya. She is imprisoned at the end of the novel after the return of Ozma.