Mombi is a character from the L. Frank Baum Oz Books series. She appears in The Marvelous Land of Oz and is alluded to other works. She also appears in Ruth Plumly Thompson's The Lost King of Oz, and a painted doppelganger of her appears in John R. Neill's Lucky Bucky in Oz.
Mombi is generally referred to as a witch, although The Marvelous Land of Oz opens with the observation that the Good Witch of the North had banned other witches from her domain and so she could be no more than a sorceress.
In the course of The Marvelous Land of Oz, she was the guardian of Tip, until he ran away. She also purchased the Powder of Life from Dr. Nikidik, and brought Jack Pumpkinhead to life with it. She assists Jinjur in her revolt in order to regain Tip. Finally, it is revealed that, in the backstory, the Wizard visited her when he usurped the throne of the Emerald City, and so she is the person most likely to know what befell the Princess Ozma.