Faustina Sachertorte is the sister of Dybbuk and the current Blue Djinn of Babylon. She took over the Prime Minister's body to get back at her father, Iblis Teer. John and Philippa save her and she repays them by rescuing their mother from becoming the Blue Djinn. She also has a crush on John.
In the fourth book, John and Mr. Rakshashas look for her spirit, while Nimrod, Philippa and Groanin look for her body, which is found, not in the wax museum as they first thought, but in the Carthusian catacombs in Malpensa, Italy . They rescue her spirit as well as her body, replacing her in her rightful place. In the process, she shares a body with John and Finlay, whom she becomes quite close with. She was able to avoid the Tree of Logic by placing her spirit elsewhere while her body remained in the palace.
Once she becomes the Blue Djinn, after the twins meet her again, she is noted to be different, affected by the Tree of Logic. John notes that she is more business-like and shows less affection to her long-time friends. As the Blue Djinn, she calls on them to stop Virgil McCreeby from finding the Eye of the Forest.