>>9 I do agree with a lot of what you said. I need to remember to separate characterization from development. >.< All four main characters have gotten characterization, Shougo and Saki having just enough to give a basic idea of what they're like, with Yuuki getting a bit more (like what you said about some of his personality traits and tendencies being shown as situations arise that elicit them). I also agree that Kaori has gotten the bulk of the development, in addition to a fair amount of characterization.
Interestingly, I think the fact that the show has more characterization than development makes the characters feel more natural and fleshed out in a broad sense than they would if they had more development than characterization. Through the characterization, you get an idea of their overall personality and demeanor, which you can apply to any of a number of different situations. From what I've seen in this and other things (both anime and non-anime shows and movies), character development usually centers around one or more specific events, and gives the viewer a good idea of what the character is like
in relation to that/those particular event(s). This tends to be more thorough than characterization, but can also be more difficult to apply on a broader scale. If done well, either way can feel very natural, but I think it's easier to have characterization that feels natural and human than it is to do the same thing with development.
In short, I think the characterization that particularly Kaori and Yuuki get makes them feel very natural and relatable (to me at least), and I think that has contributed to how endearing I find them.