J.K. Rowling uses the middle initial K in her name.
She was born Joanne Rowling, without a middle name. Before the publication of Harry Potter and the Philosopher's Stone, her publisher asked her to use two initials because the marketing team believed some boys might be less likely to choose a book written by a woman. Rowling selected K in honor of her paternal grandmother, Kathleen Ada Bulgen Rowling.
The name therefore became J.K. Rowling rather than Joanne Rowling. The initials do not represent a birth middle name, a fact sometimes misunderstood by readers who assume that Kathleen was part of her legal name.
Rowling has also published under other names. Her adult crime novels use Robert Galbraith, while later children's works such as The Ickabog were published under Rowling's own name. The initialed form remains most strongly associated with the Harry Potter series.