Nurse Spratt, now known as Leigh Douglas, was Dr. Swineheart's assistant. After the citizens of Fabletown fled to Haven in fear of Mr. Dark, Spratt stayed behind and became his mistress.
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Nurse Spratt is a bitter, angry, and vengeful yet also narcissistic person, jealous of the beauty of other Fables. Before her transformation, she has the misfortune of being the only unattractive Fable in town. This shaped her anger and jealousy at the other Fables and she believes that it is their fault. Mr. Dark takes advantage of this and promises to make her beautiful. For this, and to get her vengeance on the scorn of the other Fables who have left her behind, she joins his side.
After her transformation into a beautiful and sexy woman by Mr. Dark, Spratt shed her old identity and named herself Leigh. She has become ruthless, murderous, sadistic, egotistical, arrogant, and hellbent on having her revenge against the Fables who she believes has wronged her.
It can be inferred based on Mr. Dark's singing upon first meeting Nurse Spratt that she originated from the poem "Jack Spratt." As the song goes, "Jack Spratt could eat no fat, while his wife could eat no lean," and thus it would explain her tremendous girth. It is also possible that she was briefly married to Jack Horner, since he is the amalgamation of all the various "Jack"s within the Fables universe.