J.K. Rowling added the initial K to her name for publishing Harry Potter.
Her birth name was Joanne Rowling, and she did not originally have a middle name. Bloomsbury asked her to use two initials because the publisher believed a name without an obvious gender marker might appeal to boys as well as girls. Rowling chose K in honor of her paternal grandmother, Kathleen.
The resulting author name, J.K. Rowling, became internationally associated with the Harry Potter series. The initial does not represent a middle name given at birth, and Rowling is not formally named Joanne Kathleen Rowling in the same way a person born with that middle name would be. She has also published adult crime fiction under the separate name Robert Galbraith.