J.K. Rowling chose the initial K for her author name.
Her name at birth was Joanne Rowling, so “J” represents Joanne. The “K” does not represent a middle name that was already legally hers. Rowling selected it at her publisher’s suggestion, using Kathleen, the name of her paternal grandmother, as the source of the initial.
The author name was developed before the 1997 publication of Harry Potter and the Philosopher’s Stone. Publishers believed that a name with initials might appeal broadly to young readers, particularly boys, who might otherwise assume the author was female. Rowling has said that she did not have a middle name when she was born.
This detail is often mixed up with the later pen name Robert Galbraith, which Rowling used for crime fiction. J.K. Rowling remained her principal public name for the Harry Potter books, while Kathleen was not presented as her everyday middle name. The initial is therefore an author-branding choice with a family connection, not simply an abbreviation of her registered name.