The librarians of Sapling Village, Arianna the Roserade and Louise the Gloom.
Arianna never lets the key around her neck out of sight as it leads the way into the basement of the library, containing many, many old books and tomes. You'll never get to see it without special permission.
Louise is Arianna's faithful assistant, arranging and rearranging as necessary. She runs errands and collects new books for the library (which rivals Merlot's own collection).