William's letter.
William is an unseen side-character in EverymanHYBRID. The first mention of William occurred in the letter to Linnie that was found in the bag that was recovered near Jim Thorpe, PA.
William is speculated to be the person to whom the letter found in the hand of the dead girl in the video "Cops Checked, No Body" was addressed. That letter was first read to the audience in the video "Ashen Waste."
There is also a mention of a "William" in the original Rake creepypasta further connecting him to both Linnie and the Rake.
Jessie's grandparents were named Rose and William, as discovered in the article that was posted, regarding Rose's death. Whether Jessie's grandfather and the author of this letter are the same is unknown at this point.