When JK found out about Natalie McDonald, a 9 year old fan dying of cancer she tried to wrap up the series and let Natalie know what happens to her favourite wizard and his friends. Sadly, Natalie died before she could ever read the confidential spoilers from the author. In tribute to her memory, JK wrote Natalie into the story and even sorted her worthily into House Gryffindor so she could be with her characters forever.
She has donated so much money to charity, she lost her "billionaire" status.
Her book dedications at the beginning of each Harry Potter book, if you know what they secretly mean.