Here's a dumb theory that entered my mind; what if Stan, Kyle and Kenny aren't real? What if the show is about a schizophrenic kid named Stan Marsh and none of his friends are real, just imaginary versions of different facets of his personality?
>Kyle is the moral compass, the guy who Stan likes to think of himself as. That's why he's his best friend. In a roundabout way, Kyle represents the egotistical part of Stan's personality. He represents what Stan likes to think of himself as, the moral good guy who is always trying to do the right thing in spite of constantly being let down by the town he lives in.
>Cartman, is Stan's dark side. He represents everything Stan wishes he could say and do. After putting up with the town's stupidity for so long, he develops this darker side who does what he wants, says what he likes, consequences be damned. Eric is what bubbles underneath Stan's external demeanor, and his animosity with Kyle represents Stan trying to suppress these dark urges with his desire to still be a good person.
>Kenny represents Stan's loneliness and jealousy. Kenny is the one kid who's often ignored or considered an afterthought, which might represent Stan's feeling that he's not being heard, not even by his parents, that nobody really cares about him. And his poverty might represent Stan's insecurities which surface when he sees a well off kid with a toy that he wishes he had but knows his family cannot afford.
>Butters represents Stan's innocent, childish side. Which explains he gains more prominence in the later seasons. Throughout the course of the show, Stan goes from edgy naive brat to a jaded kid whose innocence has been tarnished after everything he's been through in South Park. Butters represents that long lost innocence.