That's a very good question. I certainly interpret Arnold Friend as an antichrist (opposite of Christ) figure. One might even say that he is satanic. The Bible tells us that Satan can appear as "a very angel of light." The first time Connie sees Friend, she thinks he is nice-looking. It is only when she gets a closer that she notices he's older than she thought, that he's wearing a wig, and that his boot is stuffed to make him appear taller. If you remove the "r" from his name, you even get "Fiend."

Who is Ellie Oscar? Perhaps he represents the devil's minions. Maybe he's there because he is creepy, and Friend can use that creepiness to his advantage as if to say, "You don't want to get him mad. Look at how much nicer I am."

Some critics have made much of numerology in the story, with the fact that Arnold Friend and Ellie Oscar and Connie all have 6 letters in their names--three people, three sixes, 666. And there is still no definite answer as to what Friend means by 33, 19, 17 (though see my answer to an earlier question, linked below).

Regardless of any link to a real person or being, it is safe to say the Ellie Oscar represents evil.