J.K. Rowling added the initial K to her published name.
She was born Joanne Rowling and did not originally have a middle name. Before publication, her publisher asked for two initials because it was believed that a gender-neutral author name might appeal to a broad readership. Rowling chose K in honor of her paternal grandmother, Kathleen.
The result was J.K. Rowling, with the J standing for Joanne and the K standing for Kathleen. The name appeared on the Harry Potter books and became one of the most recognizable author names in modern publishing.
A frequent misconception is that Rowling’s legal birth name contained the middle name Kathleen. It did not. Kathleen was the family name that supplied the additional initial for her professional byline. Rowling later used Robert Galbraith for her crime novels, a separate pseudonym with no connection to the initial K.