If anyone had ever bothered to asked him, Kenny would have told you he compared Kyle to a bull terrier. In his opinion, both were smallish, stocky and rather sturdy, both were snappy and yappy and irritating, both were fiercely loyal and bit frightening, and both had a reputation for getting riled up easily and then refusing to let things go.
Cartman never cared about being asked, and very vocally chose to compare Kyle to Mrs. Broflovski. In his opinion, both were Jewish, squat, and very ugly, both were snappy and shrill and constantly infuriating, and both had a reputation for getting riled up and melodramatic. And both were big, fat bitches who were on their fucking periods, all the fucking time.
Stan never compared Kyle to anything, because Kyle was Kyle, and to Stan, trying to find something or someone worthy enough to be compared to Kyle was like trying to find a specific grain of sand in the desert. Utterly pointless and impossible.
Kyle, for his part, never really thought he was anything all that special. His mother, and Stan, often disagreed with him, reassuring him he was so very, very special, but he'd already made up his mind, and once that mind had been made up it took the gravitational pull of the sun to change it.
Still, instead of moping about it Kyle chose to throw himself into hard work and just actions, trying very hard at school and being crowned class valedictorian whilst simultaneously clomping about to complete several (somewhat) honourable projects on the side; projects that occasionally involved firing a whale back home to the moon, joining cults, appeasing his dead grandmother, or freeing Canadian war criminals. Whatever took his fancy that week. The upshot of it was Kyle spend a good deal of his time clomping around different places, fighting against things he deemed to be unfair or unjust in some way. Stan spent a good deal of his time following Kyle about, keeping Kyle safe, noticing how all that clomping Kyle did seemed to frame his magnificent ass just perfectly.
Yet as he got older, it became more and more apparent that Eric Cartman had a point, Kyle was his mother's son. He'd had her mass of hair and her very very Jewish nose since birth, but as he aged he also began to develop her arguing skills, her crowd creating adeptness, her ability to start wars, her tendency to get slightly hysterical, her slightly shrill voice, her sort-of-East-Coast accent, as well as her height and stature. When, part way though their sophomore year, Cartman was realised from a year-and-a-half-long stint at maximum-security, maximum-intensity fat camp, he had come back to discover, to his utmost euphoric joy, that he was actually trimmer then Kyle, a fact he loved pointing out to Kyle. Repeatedly. Constantly. Incessantly.
Kyle, who had grown used to how lovely life was without Eric and his constant murder attempts, had responded to this new development, the icing on an already awful cake, rather badly; after throwing a screaming fit in the middle of the school's sports field, he'd marched home (tailed by a anxious Stan) and proceeded to make himself sick by devouring every sugar-free item (and a few of the very sugar full items) of food he could find in the Broflovski household. Stan had thought about pointing out just how counterproductive this was, but had decided against further enraging an already hysterical Kyle. The whole ordeal had ended on the living room couch, in an amazing display of gayety that saw Stan cuddling and consoling an exhausted, sick, and at this point weeping Kyle, kissing the side of his face whilst he rocked him soothingly, reassuring the stocky little redhead that no matter what Cartman did to himself, Kyle was so, so much prettier, without even needing to try.
Still, like everything in South Park, this new Cartman didn't last. Liane Cartman's new boyfriend (or client, whatever) broke up with her a few minutes after he met Eric, and so she had overcompensated for her loss by completely ignoring the camps diet plan, creating toaster pastry chocolate-mix butter bar after cakey yum-yum for her dear little Eric. Before anyone knew it Cartman was back to getting wedged in his desk, Kyle was back to happily frolicking about with Stan, Stan was back to happily watching out for Kyle, Kenny was blown up by a toaster, and everything was back to normal. Just like that.
Stan Marsh, for his part, had strolled through his teenage years on the same wave of laid-back popularity he'd had in his childhood, always the leader, always the bright ray of light shining across South Park. He grew up a perfect quarterback, tuneful and musical, toned and handsome, patent and caring, and very much a massive pussy. Still, girls (and a few guys) liked men who were massive pussies, and so Stan found himself gilded as somewhat of an idol. Not that he really gave a shit, mind you, because Kyle was right there. And who wanted girls when Kyle was there?
The only girl who'd ever really had a chance with Stan was Wendy Testaburger, but even that had fizzled out eventually. After a few years playing off-again-on-again with their relationship, Wendy finally threw in the towel half-way through their junior year. She'd bitched at Stan for months to get him to spend some time with her, and when he finally made a day in his calendar (in between the days he'd dedicated to Kyle, Kyle, Kyle, that day Kyle's busy with his family so Stan promised he'd throw a football around with the guys or something, then Kyle, Kyle, Kyle and Kyle again) they'd caught a bus up to Denver.
Needless to say Kyle hadn't been best pleased that Stan had been taken away from him at such short notice, and Stan wasn't best pleased that he was missing watching Kyle clomping about or running errands or doing homework, you know, whatever Kyle was planning on doing that day. When Stan had told him he would be spending the day with Wendy (he'd been so distraught he'd waited until he and Wendy were already on the bus to call him) Kyle had screeched a very Sheila sounding "What!" (so uncannily similar, in fact, that when Ike heard it echo into his bedroom, he'd had nightmares for weeks) before Kyle hung up the phone with a hissed "Whatever, asshat!"
Wendy had then spent the next four hours in Denver dealing with a dejected, unhappy Stan, who instead of trying to enjoy the date they were having, just sobbed over every ushanka they saw and mentioned Kyle one hundred and eight times. Wendy counted.
By the time they arrived back in South Park Wendy was passed being pissed off. She let Stan walk her to her door, she let him peck her lips, she listened to him talk about how perfect Kyle was some more, then she watched him walk away in the opposite direction of his own house, heading towards Kyle's. At that moment, Wendy realised that everyone would be better off if Stan just got his act together and removed his head from Kyle's ass so he could plug the gap with his dick instead.
Stan actually didn't notice Wendy had dumped him permanently for quite a while. He only realised what had happened when he walked past her and Token humping against some lockers about a month later. Still, it's not like he cared all that much. Kyle had recently acquired a new pair of lovely forest green cords that clung to his hips and ass just perfectly, cords that made him look so divine when he clomped. Stan found Kyle's new pants were way more interesting then Wendy.
Precisely what this laid-back, guitar strumming, animal saving pussy heartthrob found so intriguing and amazing and enlightening and joyous about a constantly snarky, occasionally hysterical, stocky little Jew no-one ever fully understood. I mean, they understood why he liked Kyle, Kyle was, after all, kind and moral and strong, he was caring and a good person. He was perfect Super Best Friend material. But they never understood why Stan was so infatuated, so in love with him.
It was just taken for granted that Park County High's star quarterback was so utterly devoted to (and perhaps a little bit obsessed by) the terrifying entity that was Kyle that Stan would follow Kyle anywhere, do anything, literally anything, for Kyle because the only thing that made Stan happy was if Kyle was happy. Kyle, for his part, remained oblivious that Stan's action were borderline obsessive. He just continued clomping about on his crusades, happy in the knowledge that Stan would follow right behind him, because to Kyle, that's just the way it always was, and just the way it should be.