The J in J.K. Rowling represents her first name, Joanne.
She was born Joanne Rowling, and “K” was added by her publisher before the first Harry Potter book appeared. Rowling did not originally have a middle name beginning with K. The initial was chosen in honor of her paternal grandmother, Kathleen Ada Bulgen, and helped create a name that did not immediately signal the author’s gender to potential readers.
Rowling has explained that she would have preferred to publish under her full name, but the publisher believed that some boys might be less likely to choose a book by a woman. After marriage, she was also known as Joanne Murray, but she continued to publish fiction as J.K. Rowling.