Ellie Oscar serves to heighten the eerie mood of the story and to provide a more physical threat to Connie.
In many ways, Ellie seems like a voiceless replica of Arnold Friend. He mirrors Arnold Friend's appearance, wearing sunglasses and a "bright orange shirt" that is unbuttoned to reveal his chest. Like Arnold Friend, Ellie at first appears younger than he actually is; only later in the story does Connie realize that he's close to forty years old but that he somehow looks like a "baby."
Ellie doesn't speak to Connie and doesn't even seem interested in Arnold Friend's plans for her. He turns up the music but doesn't look their way and instead fidgets with a radio as Arnold Friend manipulates Connie. The sight of Ellie makes Connie experience a "wave of dizziness," and Arnold Friend repeatedly belittles Ellie, who doesn't seem to mind. Ellie Oscar's presence is hauntingly vacant, as if he exists as a body without a soul. If Arnold Friend is seen as the embodiment of evil, his control over Ellie Oscar seems complete and final.
The only time Ellie speaks is to deliver a threat to support the efforts of Arnold Friend. When Connie threatens to call the police, Ellie asks, "You want that telephone pulled out?" Arnold Friend immediately reprimands Ellie for speaking at all, commanding that he "shut up" and insisting that Connie is not Ellie's "date." Arnold Friend then uses this opportunity to demonstrate a more violent side of his own personality as he tells Ellie,
"Don't crawl under my fence, don't squeeze in my chipmonk hole, don't sniff my glue, suck my popsicle, keep your own greasy fingers on yourself!" He shaded his eyes and peered in at Connie, who was backed against the kitchen table. "Don't mind him, honey, he's just a creep. He's a dope. Right? I'm the boy for you."
Arnold Friend therefore capitalizes on Ellie's physicality to present himself as a gentleman by comparison. This further exposes the psychopathic dangers that these men present to Connie.