Who sent Kathy a text message about eating skin?

In fact, there is no text message at all. This is Kathy's illusion, or imagination. Because she likes Sid and sincerely hopes that Sid likes herself, she made up this anonymous short message. In fact, there are two places about eating. Once, Kathy saw a piece of paper on the bus glass, which said eat. In another place, Kathy and Sid are chatting while eating in the canteen. When Sid finished eating, Tony called her away. Kathy was sitting alone at the dining table and saw the shape of eat spelled out by Sid with French fries on the plate in front of her.