In J. K. Rowling’s pen name, the initial “K” stands for Kathleen.
Rowling was born Joanne Rowling and did not originally have a middle name. Before publication, Bloomsbury asked her to use two initials so the name would appeal broadly to readers, including boys who might be less likely to choose a book that appeared to be written by a woman. Rowling selected Kathleen, her paternal grandmother’s name.
The name therefore became J. K. Rowling rather than Joanne K. Rowling as a birth name. She has also used other author names: Robert Galbraith for the Cormoran Strike crime novels and, in some contexts, simply Jo Rowling.
A common mix-up is assuming that the “K” is Rowling’s own middle name. It was adopted for publishing purposes and honors a family connection. The initials became one of the most recognizable author brands in modern children’s literature.