J.K. Rowling adopted the middle initial K from her grandmother Kathleen.
Rowling was born Joanne Rowling and did not originally have a middle name. When her first Harry Potter book was prepared for publication, her publisher asked her to use two initials because it was thought that a gender-neutral author name might appeal to a broad readership. Rowling chose K in honor of her paternal grandmother, Kathleen Ada Bulgen Rowling.
The resulting author name was J. K. Rowling, although she is often called simply J.K. Rowling. The initials do not stand for “Joanne Kathleen” as a birth name; Kathleen was added for the publishing name. Rowling has also used the author name Robert Galbraith for her crime fiction.
Another naming detail is that the author’s surname is Rowling, not “Rowlings.” Her first name is commonly written Joanne, while “Jo” is a familiar short form. These details explain why bibliographic records may list Joanne Rowling while books display J.K. Rowling.