J.K. Rowling used the initial K in her pen name, although she was born Joanne Rowling.
Rowling was born Joanne Rowling and did not originally have a middle name. Before the publication of Harry Potter and the Philosopher’s Stone, her publisher asked for two initials because research suggested that some boys might be less likely to choose a book written by a woman. Rowling chose K in honor of her paternal grandmother, Kathleen.
The resulting author name, J.K. Rowling, combines the initial of Joanne with the added K. The name is conventionally pronounced “Joanne Kay Rowling,” but “Kay” was not her birth middle name. This detail is why the K is often described as an adopted or chosen initial rather than evidence of a registered middle name.
Rowling later published adult crime fiction under Robert Galbraith and The Casual Vacancy under her own initials. Her name history is therefore different from a typical pseudonym change: J.K. Rowling is a constructed publishing name, while Robert Galbraith is a separate pen name.