The “J” in J.K. Rowling represents Joanne, J.K. Rowling’s first name.
Rowling was born Joanne Rowling on 31 July 1965. She did not originally have a middle name. For the publication of Harry Potter and the Philosopher’s Stone, Bloomsbury wanted a name that might appeal broadly to boy and girl readers, and Rowling adopted the initials J.K. The “K” was taken from Kathleen, her paternal grandmother’s name.
She has explained that she was asked to use two initials because the publisher believed some young male readers might be less likely to choose a book written by a woman. Rowling’s author name therefore became a publishing identity rather than a direct record of her legal birth name.
The name is often expanded incorrectly as “Joanne Kathleen Rowling,” as though Kathleen had always been part of her given name. In fact, Kathleen was added for the pen name. Rowling has also published adult crime fiction as Robert Galbraith, another deliberate author identity.