I'll be honest with you: I almost didn't read past the first volume of Rules because of this couple. Not that they were a bad couple, necessarily, but because they are so typical for yaoi: Yuki is not especially different from your average useless and pathetic uke, and Hikaru is more or less the required too-cool-for-you seme. The story was a bit more complex and realistic than that (of course, it's Miyamoto Kano afterall), but overall it was increadibly vanilla in its sensibilities--no easily offended fangirl could possibly be bothered by the way everything plays out: the uke appropriately transfers his feelings of love to the seme who popped his cherry, and the seme proves himself to have the patience of a saint while waiting for his uke to make the decision we could all see coming right from the first chapter.
So in a sense, this doujinshi should have been right up my alley as it makes things a little more spicy between this otherwise bland couple: Yuki gets annoyed with Hikaru's temporarily busy lifestyle and begins (a bit more than) a flirtation with an interested classmate of his. The classmate is more than a little odd, but he's interested and Yuki is bored, so he goes for it. Of course the cumulative effect of all of this is nothing; it's a very short doujinshi afterall.
Given the length, I certainly hadn't been expecting (or wanting) major drama, but still, it's frustrating seeing that years have passed since the first volume of Rules, and character-wise, these guys are exactly the same: Hikaru is still impossibly awesome, Yuki is still a whining brat tied forevermore to the guy he had his first sexual experience with. In the doujinshi, his one moment of rebellion against the yaoi social order of things is swept under the rug to be forgotten about as quickly and efficiently as possible (just as his past sexual transgression was quickly dealt with in Rules volume 2). But of course, it has to be this way: as uke, even at 20, Yuki must still be as needy and clingy as he was at 16, and after 4 years of putting up with a needy partner, as a good seme, Hikaru is no less tolerant and indlugent of his uke than he was during the honeymoon phase of their relationship. Relationships between people, of course, never evolve over time.
So, that was a lot of words to basically say: the story was uninteresting as it went nowhere and there was no character development either. All in all, a doujinshi that didn't need to be written.