J.K. Rowling’s first name, according to her birth name, is Joanne.
She was born Joanne Rowling in 1965. The initial “K” was not part of her birth name; it was added by her publisher before the first Harry Potter book was released. The letter represents Kathleen, her paternal grandmother’s name.
Rowling used initials because the publisher thought a gender-neutral author name might appeal to boys, who were considered an important part of the target readership. She did not have a middle name at birth, so Kathleen was adopted for the publishing name.
She has also used other author identities. The Harry Potter books were published as J.K. Rowling, while the Cormoran Strike novels use Robert Galbraith. These names should not be confused with characters or with separate writers.