Who is Great-Aunt Lucinda from The Field Guide and what is their importance?

Great-Aunt Lucinda (Lucy) never physically appears in the novel but is mentioned by the main characters. She is described as old, alive, and living in a mental asylum. It is believed that the faeries and other creatures living in the mansion may just have driven her crazy. Lucinda is the owner of the old Victorian house and allows the Grace family move in there.