Signs that a guy might be trouble:
Ah, if only Connie spent less time listening to the radio and more time reading Shmoop. (Couldn't pass up an opportunity for shameless self-promotion.)
As Connie tries to get a handle on Arnold, she realizes that:
She recognized most things about him [...] even that slippery friendly smile of his, that sleepy dreamy smile that all the boys used to get across ideas they didn't want to put into words. [...] But all these things did not come together. (77)
Arnold is a "blur," and every attempt to see him for what he really is only generates "dizziness" rather than mental clarity (94).
That's because Arnold Friend is a blend of some familiar types from literature and pop culture. He's the Matthew McConaughey character from Dazed and Confused, the guy who still hangs out at high school way after he's graduated. But he's also got qualities that a long literary tradition associates with evil – like Milton's Satan in Paradise Lost or Dostoevsky's devil in The Brothers Karamazov. (Want more devilish connotations? Take out the "r's" from "Arnold Friend" and you're left with "An old fiend.")
Like these great literary bad guys, Arnold can zero in on the weaknesses and desires of those around him – in this case, Connie's romantic fantasies. And like these incarnations of evil, Arnold's greatest tool of manipulation is a forked tongue. He's a travesty of morality, the "Friend" who isn't a friend. He keeps his promises, but his promises are all threats. Coming from his lips, the word "love" loses all of its idealistic connotations and becomes a violent and obscene thing.
No matter what Connie says or does, Arnold keeps talking – and yet he reveals nothing about himself. He never physically coerces Connie to join him, but his words have the same force and pull as the actions he only threatens to take:
"Soon as you touch the phone I don't need to keep my promise and can come inside [...] anybody can break through a screen door and glass and wood and iron or anything else if he needs to, anybody at all, and specially Arnold Friend." (116-118)
Death or evil incarnate posing as an ordinary man: this is the mess of contradictions that makes Arnold Friend so terrifying and so unforgettable.