J.K. Rowling, the prolific author of the hugely popular Harry Potter series, has used her writing expertise for good, sending a handwritten letter to 15-year-old shooting survivor Cassidy Stay.

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Ms Stay, a teenager from Houston, Texas, was the lone survivor in a mass shooting that killed her entire family in July.

The letter is reportedly written from the perspective of Rowling’s character Professor Albus Dumbledore, whom Stay quoted at a memorial for her family.

“In The Prisoner of Azkaban, Dumbledore says: ‘Happiness can be found even in the darkest of times if one only remembers to turn on the light,'” Ms Stay said.

Thanks to an internet campaign raising awareness about Ms Stay’s situation, Rowling was inspired to send the teenager a letter from the fictional Hogwarts headmaster.

Ms Stay, who called 911 after her aunt’s ex-husband went on a shooting rampage, has received an outpouring of support after exhibiting bravery and grace under immense pressure.

Watch her touching memorial speech below: