What are the signs that foreshadow Arnold's true character?
Although the main character does not realize it at first, Arnold Friend is not what he appears (i.e., a rakish adolescent boy a la James Dean or Elvis). The story focuses on the contrast of appearance and reality. One aspect of this is the romance of youth (especially in the summer), and how a dreamy, teen-aged, rock-and-roll-fueled view of the world is both perfectly natural and ultimately dangerous. The man character's failure to recognize Arnold for what he is, and her failure to act, ultimately lead to what the reader assumes is her death.
Arnold friend has several physical characteristics that, when examined, let us know that he is not what he seems. Although at first glance he seems handsome and normal, if rather bad-boyish, a closer examination reveals that he is actually wearing a costume. We assume this costume is meant to attract young, vulnerable girls. Arnold has a false tan (done with makeup), and is actually much older than Connie first thought. He stuffs his boots to appear taller, his hair has an odd look (as if it is a wig), and his grasp of teenage lingo makes it seem like he doesn't understand what the words actually mean. His car, which at first seems flashy, is actually frightening, too. It is emblazoned with strange messages that seem to have darker meaning (the numbers, for instance, may represent people he killed, and also add up to a number with a sexual connotation).
Many critics view Arnold Friend as a representation of the devil (his name can be read as "An Old Fiend," and one could argue that his stuffed boots hide cloven hoods and his wig a pair of horns). This concept--that evil is hiding everywhere--reinforces Oates' theme of appearance versus reality.