"Sheriff Keller told Alice the letter to the Riverdale Register was written by someone different than the letter to Betty, and I have a theory that it was Kevin, and that Kevin has also been the one calling Betty. He has a motive (being upset about Betty going to his dad about his cruising the woods), he would know that Betty would tell Archie about the conversations, and he would know that she loved Nancy Drew as a child. It would also explain the 'I don’t like sharing you' line, because he and Betty are best friends. Kevin got over their fight waaaay too quickly, and the writers said there would be more of a storyline for him this season. Maybe this is it."