In real life, Natalie McDonald was a young Canadian fan of the novels with a terminal illness, leukaemia. A friend of her mother's wrote to J. K. Rowling to ask her to write to Natalie. J.K. Rowling did not receive the letter until after a holiday, and though she admits to fearing it was "too late", wrote back, telling Natalie what happens to each of the main characters, only to find out from Natalie's mother that she had passed away. The two women began a friendship, and Natalie was thus added to the books (into Gryffindor, the house of bravery), as a tribute. Her mother kept the contents of the letter secret until her other two children finished reading The Deathly Hallows.
This tribute can be found on page 180 of the American paperback version of Goblet of Fire, with Nearly Headless Nick applauding as she sits at the Gryffindor table. In the British edition, the tribute is located on page 159.