Thripp is a "short, odd-looking woman" whose clothing is compared to something rescued from a rag bag: wrinkled, stained, and untidy. She describes herself as a "not very powerful witch." She had earned the nickname "Crazy Annie" from mean children and Professor Childermass's brother, Peregrine.
Following Perry's death, his brother Roderick comes to Maine with his young friends, Johnny and Fergie, to live in Perry's house and claim Perry's inheritance. It's during their extended summer visit that Perry's ghost appears to Johnny and tells him "Crazy Annie has the key."
She lives in a "squat shabby bungalow with a gambrel roof and screened porch" with old license plates and red taillight reflectors nailed to trees in the yard. Her kitchen contains a large cardboard carton with heaps of keys of all kinds and shapes. The key Perry speaks of, however, is none of these but rather one of Thripp's first magical creations: an enchanted piece of costume jewelry made of twinkling rhinestones.
It is unknown whether the character was inspired by any one person, though the name "Ms. Anna Thripp" sounds suspiciously similar to "misanthrope," a person who hates or distrusts humankind.