So much so that Fabian's parents placed the letter next to their 12-year-old son after he lost an eight-year battle with acute lymphoblastic leukemia on Britain's Remembrance Day.
"I want Kate to know it wasn't just a letter, it was a message that meant so much," father Darrell Bate told London's Mirror. "It has stayed with him and it's staying with him now he's departed."
Kate, who had only been the Duchess of Cambridge for about six months at the time, wrote to Fabian on Oct. 13, 2011, after she and Prince William met him in the hospital about a month beforehand.
"Despite the enormously demanding course of treatment you are undergoing, I was so touched by your strength of character, and delighted to hear the news that one of your big sisters will be able to donate bone marrow to you later this year," read Kate's later, typed on St. James Palace stationery. "I will keep my fingers crossed that your health goes from strength to strength over the months ahead."
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Sadly, Fabian, who was first diagnosed with cancer in 2006, died at home in Hampton Court, Surrey, surrounded by his parents and three siblings.
His son's letter "is the only one in the world, it has that uniqueness—it is very personal and special," Darrell also told the Mirror. "It was synonymous with what Fabian went through, he would not have got it otherwise. It would be amazing to think that Kate and William knew that the letter she wrote three years ago is now sitting next to his body in rest."
Per the Mirror, a spokesman for the royal couple said, "The Duke and Duchess are sad to hear the news and their thoughts are with Fabian's family."