If the best way I can think of to describe Touya is dark sexiness, what leaps to mind when I think of Yukito is cute and sweet. While Touya is intense and generally acts very serious, Yukito is lighthearted and seemingly as open as Touya is reserved. Physically, he's substancially shorter than Touya, has very pale skin, amber eyes and really cute, bouncy grey hair. He's not exactly feminine or androginous, or at least not so much as other CLAMPish bishounen (TB Subaru-kun, anyone?), but his delicate features transmit a certain frailty, especially when compared to Touya's sharper, stronger looks. As far as I'm concerned, he's the perfect complement to Touya, both physically and emotionally ...but I'll get into that in another section. ^_^
The story on Yukito is that he and Touya met in their first year of high school, because Yukito had just moved to Tomoeda to live with his grandparents (not that one has ever actually /seen/ them... in fact... one could almost question if they actually exist... innocent look). Given Touya's personality, it's very revealing that he and Yukito got along very well, very quickly; enough that, as Sakura once explained, Touya "brought him home" to introduce him to Fujitaka and Sakura.
So... (says a shounen ai infested mind) Touya brought Yukito home to meet his family, did he? Kawaii! Er--anyway, even if you don't read anything into the way Sakura phrased it (after all, at the time she didn't know the true nature of their relationship... and in all fairness it was probably too soon for them to have come to any sort of understanding), the way she says it certainly seems to imply that Touya bringing anyone home was very rare indeed. He sensed, he knew, that there was much more to Yukito than met the eye (cough spoiler! cough), that in his own way, the other boy was as different as he himself was. And Yuki's kindness and smile were probably all that was needed to bridge whatever gap there might have been between them. They started spending more and more time together, until becoming pretty much inseparable. By the time CCS starts, Yukito seems to be a permanent fixture of the Kinomoto household, even spending the night to, um, study ^.~ on several occasions. He seems to spend every moment he can with Touya, even visiting him at work or helping out the soccer team (Touya is in the soccer team). For his part, Touya has a busier schedule than Yukito, but his protective, even jealous attitude makes his feelings nore than clear enough.
So what was it about Yukito that made Touya open up so completely?
Yukito's most characteristic trait is his smile, and the friendliness and kindness that go along with it. He wears his heart on his sleeve, and is the sort of person that finds the good in any situation (remember the contest he and Sakura lost, and how getting a PEN as a consolation price was enough to make him happy?). He accepts everyone, and nowhere is it clearer than in the way he treats Sakura and her friends. I'm quite sure that the reason she's crushing on him is that he treats her not as little kid, but as a person.
One would think someone like him would have a million friends, ne? And yet his social life is, shall we say, somewhat limited. I get the feeling that the reason is that, despite being so friendly, he's really quite shy. Not a cripplying sort of shy, just a little bit shy. Think about it, he always seems a little surprised whenever anyone shows him kindness, even Sakura. Like he doesn't know he's a wonderful person that brightens the day for anyone. I also think that's the reason he gets along so great with kids. I think he relates to their innocence, that they put him at ease in a way most people his own age don't.
But, again, the shyness theory comes mostly from circumstancial evidence, and it's not something he couldn't get over if he really wanted to. He doesn't really seem to want to broaden his social horizons. The thing about Yukito is that his life is very Touya-centric, and he obviously likes it that way. Take the fact that he doesn't belong to any clubs. He's a smart guy, and he's good at sports; there are any number of clubs he could join; in fact, he helps out the soccer team, the archery team and the basketball team. Of these, the one he seems to have by far the strongest ties to is the soccer team... and that's probably only an excuse to be near Touya during practice. The same goes for afterschool jobs. Let's be honest here, it's not like he HAS to work (his grandparents wouldn't spend so much time cruising the world if they weren't moderately well off). It's safe to say that for Yukito, the main incentive for having the sporadic afterschool job is having the chance to spend even more time with Touya. Fnally, there is the fact that aside from Touya, his social life seems to consist mainly of ten year olds. That's not quite true, it consists of one particular ten year old and her friends. A ten year old who just HAPPENS to be Touya's little sister...
The fact is that Touya is obviously the most important person in Yukito's life. His life revolves around To-ya... and so does his heart.