The Morning Star
For his great love, Byakuya defies his status and family expectations. He chooses her again when he adopts her abandoned sister into an unwelcoming household. With promises kept, he assumes the shackles of nobility once again, swearing to his parents' graves that never again will he disrespect their traditions. So when his sister in law, and by law, is marked for execution, what is a man to do?
Conflicted Beginnings
Over the forty years since I was adopted by the Kuchiki family, not even once has that person truly looked at me.
Byakuya never gives his side of the story on this, so we'll have to play the assumptions game. Since he went through all the trouble of finding and adopting her, at least that shows there was no love lost between him and Hisana. It does beg the question, however, if he ever saw Rukia as anything other than a promise he made to his late wife. It wouldn't be too much of a stretch to say that seeing Rukia even hurt him because she was a reminder of what he had lost. His denial of affection toward her may be a result of a damaged heart and the rallying of his family pride.
In plot context, Byakuya's decision to uphold Rukia's execution represents both great strength and weakness. For him to deny his promise to Hisana and lawful tie with Rukia for the sake of the system, he endured much suffering. However, in many ways it was the easier path for him, to be the symbol of stability throughout the ordeal, because defiance would surely make him lose face and dishonor the oath he swore to his parents. He couldn't choose love this time, not wanting to hurt, but hurting all the same. The revelation of this conflict still tears me up to this day.
His Real Pride
After Ichigo finally makes Byakuya realize how misguided he had been this whole time, it's as if someone turns on the "big brother" switch in his brain. In spite of great injury he shields her from Gin's attempt on her life, and in that moment their bond is forged in gold. After all the pain he had caused her, Rukia instantly stamps out those feelings and clings to him in futile protection. In another quiet display later, as he explains the true circumstances of her adoption, Byakuya intentionally seeks her hand before offering his heartfelt apologies.
In forty years, as Rukia described, their interactions were always awkward and forced. Yet somehow, in a matter of moments those decades are lost to the sincerest display of "family." Byakuya doesn't suddenly transform into a warm, loving brother, but it is clear that the tensions of the past are gone and buried.
In his bout with Zommari Leroux, the 7th Espada, Byakuya refuses to state his name at any point. Even as the enemy drones on about the injustice of shinigami slaying Arrancar, to which the old Byakuya would probably have made some snip about them deserving it, he simply replies:
When did I say, "As a shinigami, I am slaying you"? The only reason I am slaying you is because you had the audacity to raise your blade against my pride [Rukia].
The great Kuchiki Byakuya proclaims the sister he was prepared to execute as his pride--one of the most profound statements made in the series. What's more, the type of "pride" to which he refers isn't how one talks about the achievements of a prodigy. No, I think Rukia represents not only the two instances he defied tradition and chose for himself, but also that he would do it again if it was for her. That dedication, not his name or his position, makes him who he is. And that is what I call character development.