J.K. Rowling’s middle initial K traditionally represents Kathleen.
Rowling was born Joanne Rowling and did not originally have a middle name. Her publishers wanted a two-initial pen name for the Harry Potter books, partly because they believed a gender-neutral presentation might appeal broadly to readers. Rowling chose K in honor of her paternal grandmother, Kathleen Ada Bulgen.
The name therefore became J.K. Rowling rather than Joanne K. Rowling as a record of a birth name. The author has also used other names in publishing: the crime novels featuring Cormoran Strike were released under the pseudonym Robert Galbraith.
A common mix-up is assuming that the K stands for Rowling’s own given middle name or for “Katherine.” Neither is the standard explanation. Rowling has said that Kathleen was selected because it was her grandmother’s name, making the initial a publishing choice with a family connection rather than an original legal middle name.