I'm way late to this conversation but I'll put my two cents in anyway. I think Kyle is supposed to be an idealist who is trying to make things "the way they should be." I think he naturally evolved to be this character to balance Cartman out. Cartman is chaotic and, when he sets out to do something, leaves a trail of disaster wherever he goes. Kyle often swoops in towards the ends of episodes and restores order in some way.

One scene in particular sticks out during the fish sticks episode. He tries to convince Jimmy to stand up to Cartman. Meanwhile, Craig (who I think is supposed to or, at least, seems to represent a realist/objective point of view) just tells Jimmy to take the half Cartman is offering. When Kyle argues with Craig that Cartman shouldn't get half because it isn't right, Craig just says "Yep, and if I had wheels I'd be a wagon." AKA doesn't matter if it's right or not, it's just the way things are.