The Muse had a few ideas about these two, and indicated that the balance of the twenty was available for digging out. So, I dug.
Tite Kubo's, not mine, not for profit.
1. If, before that damned Ichigo Kurosaki showed up in his life, you asked Byakuya Kuchiki to name five people he found attractive, Renji Abarai's name would have been absent from that list. (Your own would have been on a toe tag, and the cause of death listed as "Hypothermia." But that's another story.)
2. If, during the same period, you asked Renji Abarai to name five people he found attractive, Byakuya Kuchiki's name would have been absent from that list. This is not because Renji didn't find Kuchiki-taichou toothsome. It was simply that Renji's life as a Rukongai gutter rat taught him to deal only with what's in front of him, and he saw Kuchiki-taichou as above him ... then.
3. If, during the same period, you asked both to name five people they were likely to be sleeping with five years hence, neither's name would appear on the other's list had you expanded the number of items to five hundred.
4. If, during the same period, you asked both of them to make a wish list, you would have seen both their names.
5. Of course, you would have been allowed to see Byakuya's wish list only after you pledged secrecy, pledging your life, your fortune, your sacred honor, and the life of your first-born child as hostage to your silence, from a position on your knees, where you were held by kidou. You would have been released after the pledge, but not until the head of the Kuchiki clan heard you give a gulp of horrified realization that he could indeed leave you there on your knees until you starved to death. (Although he probably would have had the servants carry you off to a dusty corner to do that; why should he be put to the bother of explaining you to his guests, or being troubled to step around your starving pitifulness his own high-born self?)
6. Renji, on the other hand, would have shared his bottle of saké with you, thrown an arm across your shoulders, and breathed a saké-laden whisper in your ear: "Anybody else hears about this, and you're shish-kabob on the end of Zabimaru. You got it, pal?" When Renji heard ya gulp, because ya understood that he had th' power ta do exactly that anytime he wished, his arm would'a fallen from yer shoulders, and he would'a poured ya another hit o' his saké. No hard feelin's, pal, just the way it is. –The two men are quite alike in their practicality, if worlds apart in their approach.
7. Byakuya Kuchiki, for his part, is still not quite sure how this - affair - with his subordinate happened. Really, he says to himself, none of my other lovers have been anything like Renji Abarai. Those tattoos, that hair ...
8. But then he remembers that that hair is quite natural, the rug matching the curtains as it were, or that if it isn't, Renji goes to a great deal of painful trouble to make it seem as if it is, and the clan head's cheekbones get a little pink. Honestly ... how many people would dye their leg hair for verisimilitude? And as for other sites on that muscular body ... no, those don't really bear thinking about, let alone listing here.
9. Anyone who enquires as to the cause of this empinkening on the part of Byakuya's cheekbones gets ferociously snubbed ... unless it is the possessor of the matching-all-over hair. Then other things entirely ensue.
10. Renji, for his part, is not quite sure how he was granted access to an armful of lethal elegance pretty much any time he wants it, but he is not going to do anything to screw that up emotionally. He could give a rat's ass about Byakuya's money and power; Renji knows that he can survive without either.
11. Renji also knows that he can survive without Byakuya. He simply wishes to avoid that necessity.
12. Renji's motivation for this control of his own behavior would astonish Byakuya if he knew of it. Byakuya has been courted (in many senses of that word) for his position, his money, and his power through the whole of his conscious life. He simply assumed that Renji was out for what he can get, because everyone else, including Hisana, was too.
13. So having someone court him for himself, for simple liking of the person behind the mask, is an entirely new experience for Byakuya. Thus far the outstanding example occurred one evening when he returned to the mansion after he had taken a painful fall during a Living World patrol. Byakuya found that Renji had worked with his butler (Renji's immediate acceptance by his staff puzzles, annoys, and slightly alarms Byakuya) to provide him with a much-needed space for rest and recuperation, while still allowing him to get his work done. Renji spent a great deal of time the next three days ferrying papers between the Kuchiki mansion and the Sixth Division, and did not participate in the steam baths and massages he had so painstakingly set up for Byakuya.
14. Byakuya is still processing this one. After all, he is used to receiving gifts of wonderful experiences he is expected to share with the giver, or gifts which are quite expensive, but whose cost was not an inconvenience to the giver. To have someone arrange Byakuya's life in a pleasant fashion for three days when he needed it, and not claim a share in those pleasantries, is unusual. To have that same someone put himself to quite a lot of work and trouble to ensure that Byakuya could recover when his very soul felt battered (and he, of course, had shared this with no one) is exceptional.
15. Could it be that Renji Abarai loves him? That the redhead's kindness is for Byakuya himself, not for his position, for his title, or for his wealth? If so, that will be a first in Byakuya's life, an experience so far outside the rest of his history that he can barely ask this question. And answering it? Pfft. It seems, though, as if it might be safe to thaw a bit in Renji's embrace.
16. Renji, who is far more interpersonally adept than Byakuya, has noted this thaw with great happiness, not for himself but for Byakuya. Renji is wise enough not to comment on the fragile new growth, but to encourage it gently, without seeming to do so.
17. That said, Byakuya would be quite surprised to find that Renji has a list of nineteen items whose title, written in the redhead's rough-and-ready kanji, is, "Stuff I Will Not Put Up With From Anybody." Item #7 is "Being kept on the bottom."
18. This resolution on the part of the redhead is responsible for the broadening of Byakuya's horizons, if you will. As the Kuchiki clan head was not exactly encouraging of this effort, it required took gentleness, patience, tact, cunning, and judicious application of sneakiness to get there. Fortunately, Renji has a large supply of all five.
19. Upshot #1: Renji Abarai knows damn' well that he is never going to get over Byakuya Kuchiki. Upshot #2: Byakuya Kuchiki is beginning to figure out that he is never going to get over Renji Abarai.
20. Upshot #3: That's all right with both of them.