As it happened, my first elementary school had not one, but three versions of "that kid." Specifically, they each filled the roll of "that gross kid." Their names were Hannah, Justin, and Anthony... and although they were disgusting in different ways, each of them seemed to be in constant competition to see who could provoke the most revulsion from their peers.
Hannah was a dark-haired girl with a perpetual glower on her face and a sullen, stubborn demeanor. She had a reputation for being somewhat antisocial, and probably could have been mistaken for a crotchety old woman if she hadn't been in first grade. Hannah's claim to repugnant fame stemmed from the fact that every so often, she'd wander around the playground in search of discarded food, which she'd eat right off the ground. It didn't matter if the fare had been thoroughly stomped on, and it wasn't the result of any malnutrition, either. In fact, there were days when Hannah actually forewent consuming her own lunch in favor of seeking out others' leftovers.
Justin was a more amiable fellow, but his particular form of foulness was also much more extreme. For no reason other than to amuse himself, Justin would do anything he could to disgust people, from jamming things up his nose to sticking his head in the toilet. To make matters worse, it seemed that most of the time, Justin didn't understand what he was doing at first. There was one occasion, for instance, that he tried cleaning his dirty fingernails with his teeth... and when this was pointed out as being repulsive, he laughed aloud and made a big show of eating (and apparently enjoying) his own fingernail gunk.
Anthony, however, was the worst of all.
If you've seen the television show "Louie," then I want you to picture the boy named Never. Everything about him - from his appearance to his attitude - is nigh-on identical to how Anthony was. If you haven't seen the show, though, try to imagine a fat child with absolutely no shame, an expression of oddly disquieting innocence, and a tendency to cause gag-inducing circumstances without so much as batting an eye. Anthony's worst-ever offense occurred one terrible afternoon as our class was watching a movie, when he was seen reaching beneath his chair, chipping off dried boogers, and then openly eating them.
When told that his behavior was revolting, Anthony replied: "No, it's okay. I put them there."
TL;DR: Floor food. Awful antics. Booger buffet.