J.K. Rowling’s middle initial is generally said to represent Kathleen.
Rowling was born Joanne Rowling, without a middle name. When Bloomsbury prepared to publish her first Harry Potter novel, the publisher asked her to use two initials rather than her full first name. The marketing decision was based on a belief that young male readers might avoid a book written by a woman.
Rowling selected “K” in honor of her paternal grandmother, Kathleen Ada Bulgen Rowling. She therefore became known publicly as J.K. Rowling, even though Kathleen was not her legal middle name at birth. The initials quickly became one of the most recognizable author names in publishing.
The name can cause a separate mix-up: Rowling is sometimes called “Joanne Kathleen Rowling” as though that were her original full birth name. “Joanne” was her given name, while “Kathleen” was added as the professional middle initial later.