From Lorraine's point of view, Norton Kelly is TROUBLE. She says that he "has eyes like a mean mouse, and he's the type of kid who thinks everyone's trying to throw rusty beer cans at him". He's a fighter, and dangerous because in addition to his belligerent personality, he is big, "even bigger than John". Lorraine says that Norton and John hate each other.
According to Lorraine, Norton is a thief and "a social outcast". He earned his reputation in his freshman year of high school, when he was "caught stealing a bag of marshmallows from the supermarket". His crime was written up in the newspapers, and from then on, everyone called him "The Marshmallow Kid".
Norton is the one who started cheating on the telephone marathons Lorraine and John were having, peeking when it was his turn to pick a name from the phonebook at random so that his finger would fall on a woman's number instead of a man's. The theory was that "you could always make a woman talk twice as long as a man", but in reality, this didn't work for Norton because he was so obnoxious that "it didn't matter whom he spoke to on the phone...they all hung up" (Chapter 4).
Lorraine's assessment of Norton is for the most part accurate, as he proves later in the book when he crashes the illicit party given by John and Lorraine at the Pigman's house. Norton crudely breaks Mr. Pignati's precious pig figurines, and gets in a violent fight with John (Chapter 13).