What middle name does J.K. Rowling’s pen name use in “J.K.”?

The “K” in J.K. Rowling stands for Kathleen.

Rowling was born Joanne Rowling, without a middle name. Her publisher asked her to use initials for the Harry Potter books, partly because a gender-neutral author name was thought likely to appeal to a broad readership. Rowling chose “K” in honor of her paternal grandmother, Kathleen.

Her published name therefore became J. K. Rowling, even though Kathleen was not part of her birth name. The initials are often mistakenly expanded as “Katherine” or treated as if they were her original legal initials.

Rowling has also written under the name Robert Galbraith for her crime novels. That separate pseudonym has no connection to the “K” in J.K. Rowling. The distinction between her birth name, publishing name, and crime-fiction pseudonym is a common point in biographies of the author.