The “K” in J.K. Rowling’s pen name came from her grandmother Kathleen.
Rowling was born Joanne Rowling, without a middle name. When Bloomsbury prepared the first Harry Potter book for publication, the publisher believed that a name with initials might appeal broadly to young readers and avoid revealing the author’s gender. Rowling chose the initial K in honor of her paternal grandmother, Kathleen.
The author therefore became J.K. Rowling in public, even though Kathleen was not part of her birth name. This origin is sometimes confused with the name of her mother, Anne, or with the mistaken assumption that the initials represent “Joanne Kathleen Rowling” as a full legal name.
In later life, Rowling has also published under the name Robert Galbraith for her crime novels. That separate pseudonym was not involved in the creation of the J.K. initials, which were established for the original Harry Potter publication.