First off, in order to realize that they're perfect together, you must know them both as seperate units. We'll start with Mitsui Hisashi, MVP, star 3-p shooter, and ex-juvenile delinquent and gang member. Sexy to a fault, I might add, and he knows it. One flash of his oh-so-drool-worthy grin and many girls find themselves melting (and in some cases, guys too). However, it was not always like this.
Mitsui had been the MVP of his junior high, but there was an acciedent. This part I'm not too clear on, but I know that after the accident, he couldn't play basketball for a while. I'm not sure if they thought he'd never play again. But I do know that he left then, for two years, becoming a gangster and growing his hair out long. It was in this foggy period of his life that he gained most of his scars, including the intriguing one on his chin. When he finally came back, he made a promise to Anzai-sensei that he'd give up fighting. He cut his hair short, and joined the team. But before all this, I do know that he slapped Kogure. Hard enough to knock his glasses off, in fact. For this he feels guilty. After all, Kogure's been his best friend since middle school, and was the first person to accept him when he got back. It pained him, since Kogure is such an open, honest person, not afraid to show his feelings. He seems like an aloof hottie, but most think of him as tender and caring for those close to him, and that includes Kogure. It may not be very easy for him to show his feelings, but that doesn't seem to be the case with Kogure, since he's usually the one, in most fics, to initiate the relationship between the two of them. He's sexy, it's true, and something of a wild man. All this is balanced out perfectly by Kogure's calm and exceedingly gentle persona.
Kogure Kiminobu is the sweetest guy to ever walk the anime world (yes, Tsuki knows she exaggerates). In fact, I think it was Ruby-san that once remarked how much like a woman he was. It's no wonder many think of him as gay. Kogure approaches everything with dogged determination and optimism, rarely allowing himself to look at things negatively. He's not naive, though. He's very aware that the world is not a place where everything turns out right; after all, his best friend was swallowed up into a life of pain and delinquency because of a basketball injury, and Kogure was forced to watch him disappear into it. However, when Mitsui came back and finally joined the team, Kogure held nothing he'd done in the past against him, showing a caring and forgivness that ran deep within him. In a way, it sort of helped heal Mitsui, to know that his friend didn't hate him because of the things he'd done. And for all his wildness, Kogure is always there behind him to contrast with it, add a calm element to him that balances them together perfectly. Like Yin and Yang, if you want to be symbolic about it. Mitsui is yin, dark and absorbing you into him and looking into your soul with meticulous, wary alert, if you will. Kogure is yang, light and pentrating to your very core with his caring attitude. (Though, granted, I do know that tradiationally women are perceived as yin and men as yang, and it would be better to make Mitsui yang...however, Kogure is in no way dark. So there's my logic in it)