J.K. Rowling’s middle initial in her professional name is K.
Rowling was born Joanne Rowling and did not originally have a middle name represented by K. When Bloomsbury prepared to publish her first Harry Potter novel, the publisher believed a name with two initials might appeal to a broad audience. Rowling chose K in honour of her paternal grandmother, Kathleen.
The resulting author name was J.K. Rowling. The initials stand for Joanne Kathleen Rowling, although Rowling’s official birth name was Joanne Rowling. She has also been publicly known as Joanne Murray after her first marriage, but her principal author name remained J.K. Rowling.
The initial is not the same as a secret identity or a character reference. Rowling later used another pseudonym, Robert Galbraith, for her crime novels. The distinction between J.K. Rowling, Joanne Rowling and Robert Galbraith is therefore useful when identifying her different publishing contexts.