J.K. Rowling added the middle initial K to her author name before Harry Potter was published. The K represents Kathleen, the first name of her paternal grandmother.
Rowling was born Joanne Rowling and does not have a middle name on her birth certificate. Bloomsbury asked her to use initials because the publisher thought young boys might be less likely to choose a book written by a woman. The resulting author name became J.K. Rowling.
The initial is sometimes mistakenly expanded as “Katherine” or treated as Rowling’s legal middle name. Neither is correct. Kathleen was selected as a tribute to her grandmother, while the “J” stands for Joanne.
In later publishing, Rowling used other author identities. Her crime novels appeared under Robert Galbraith, and the Harry Potter credit continued to use J.K. Rowling. The initial therefore belongs to her public author name, not to a previously hidden middle name.