Ruth Black is the unofficial tour guide who lives at the top of the Empire State Building. Oskar meets her with Mr. Black. She either sleeps on the observation deck itself or in one of the storage rooms when it is cold. Though the situation seems immediately strange both to Mr. Black and to Oskar, she explains quite simply that no one has ever told her to go home.
When he was alive, her husband used to shine a spotlight from the city up to the Empire State Building so that she could see it from above. When he died, Ruth came back to the top of the building and has remained ever since. She lives (allegedly) off of food from one of the two snack bars at the top or occasionally one of the "young men" will bring her food.
She does not provide any clues to the mystery of the key, and after leaving the Empire State Building that day, Oskar's companion (Mr. Black) announces he's finished with the search.