The “K” in J.K. Rowling’s pen name came from the name Kathleen.
Rowling was born Joanne Rowling and did not originally have a middle name. Before publication of Harry Potter and the Philosopher’s Stone, her publisher asked her to use two initials because research suggested that young boys might be less likely to choose a book written by a woman. Rowling selected Kathleen, her paternal grandmother’s name, as the second initial.
The resulting author name was J.K. Rowling, although she is commonly called Joanne Rowling or Jo Rowling in personal and professional contexts. The initials do not stand for a secret full name such as “Joanne Kathleen Rowling” given at birth; Kathleen was adopted for her publishing identity.
The name choice also explains why the United States editions used “J.K. Rowling” even though the British-born author’s registered first name was Joanne. The initial-based name became closely linked to the worldwide Harry Potter brand.