The middle initial in J.K. Rowling’s published name is K.
Rowling was born Joanne Rowling, without a middle name. When Bloomsbury prepared Harry Potter and the Philosopher’s Stone for publication, the publisher asked her to use two initials because it was thought that a gender-neutral presentation might appeal more broadly to boys. Rowling selected K for Kathleen, the first name of her paternal grandmother.
The author’s name is therefore written as J. K. Rowling, with J representing Joanne and K representing Kathleen. She is often called Jo by friends and family. The spacing and punctuation of the initials are part of the familiar professional name used on her books.
A common misconception is that Rowling’s full birth name was Joanne Kathleen Rowling. Kathleen was added for the publishing identity rather than appearing on her original birth record. After her marriage to Neil Murray, she was sometimes known as Joanne Murray, but her books continued to use J.K. Rowling.