J.K. Rowling added the middle name Kathleen to her published name.
She was born Joanne Rowling, without a middle name. For publication, Bloomsbury asked her to use initials, and she selected K for Kathleen, her paternal grandmother's name. The resulting author name was J.K. Rowling.
The initials helped create a name that did not immediately signal the author's gender to potential readers. Rowling has explained that she had no middle name at birth, so the K was chosen specifically for the publishing identity.
Her legal name later changed through marriage, but J.K. Rowling remained the professional name associated with the Harry Potter books. The use of initials is therefore a publishing decision rather than evidence that “K” was part of her original birth name.