In Bleach, black-robed Soul Reapers use their zanpakuto to purify Hollows and escort the souls of the dead to the afterlife. But they can love and have families like anyone else.
Rukia Kuchiki is the first Soul Reaper to appear, and she set the tone for her kind as a consummate professional, dedicated to her duty with no fear or hesitation. Her big brother, Captain Byakuya Kuchiki, is the archetypical cold professional, and at first was an antagonist who had a big problem with Ichigo Kurosaki. However, Byakuya soon changed his ways after meeting Ichigo, embodying the entire Soul Society's change for the better.
Rule Of Law
Unlike a few of his quirky compatriots, Captain Byakuya Kuchiki is a model Soul Reaper, much like Rukia is. He is also the head of the noble Kuchiki family, meaning he has a lot of responsibilities in his everyday life. Her's up to the task: not only is he highly skilled and experienced, but he has a rigid way of thinking that allows him to run his Squad and noble house alike. Byakuya Kuchiki strongly believes that following the law will maintain peace and order, and that this is his ultimate responsibility.
Chaos, impulse and improvision don't exist in his life, and only twice had he broken the rules. The first time was when he married the commoner Hisana into the family, and the second time was when he adopted Hisana's estranged sister Rukia into the family as Rukia Kuchiki (Hisana had already died by then). In short, Byakuya represents the epitome of what a law-abiding, disciplined Soul Reaper and nobleman should look like, and that is his initial role in the story.
When Byakuya is introduced, he's an antagonist who is motivated not by evil or selfish ends, but duty. Byakuya was never a true villain, but his fanatical views on the law meant he had to arrest his own sister and take her back to the Soul Society for court, destroying anyone who interfered. Ichigo Kurosaki tried to get in Byakuya's way, and he was struck down at once. This clearly established Byakuya as an obstacle, both in principle and as a fighter, and gave Ichigo a clear goal to reach for. Now it was Ichigo's mission to train up, storm the Soul Society and defeat Byakuya both on principle and on the battlefield. This rivalry was the driving force of the Soul Society arc until the very end, when Captain Sosuke Aizen outed himself as the ultimate betrayer and schemer.
What He Fights For
Fighting Ichigo planted the seed of change in Byakuya Kuchiki. This Captain is not a cold-hearted monster; he took no pleasure in upholding Rukia's scheduled execution. Instead, Byakuya defines himself with his responsibility, leadership and rule of law, something that Ichigo seemed to threaten. But after Ichigo fought Byakuya to a draw, the Captain was grudgingly impressed by Ichigo's stubborn desire to rescue Rukia, and Byakuya demanded to know why Ichigo would go this far. Ichigo gave his simple answer: that personal loyalty and friendship are more important to him than any law. Byakuya began to warm up this idea, and tried to get Ichigo to see his side at the same time. If Byakuya didn't uphold the law, he argued, then who would? Someone had to, right? Not necessarily.
After Aizen extracted the Hogyoku from Rukia and fled to Hueco Mundo with Captains Ichimaru and Tosen, the badly injured Byakuya underwent recovery, then emerged as a new kind of Captain. He realized that his strict, nearly blind obedience to the law had robbed him of the warmth and comfort of family ties, and he had taken Rukia for granted and thus defied his late wife's wishes. Rukia had been precious to Hisana; now she was precious to Byakuya too. He was ready to take responsibility for that, and when the self-righteous Arrancar Zommari tried to execute a helpless Rukia, Byakuya stepped in not out of duty, but as a protective big brother.
Byakuya symbolizes the change that affected the Gotei 13 after Ichigo's rescue mission: that the Soul Reapers now fought for friends, family and justice, not simply out of duty. This change affected Rukia, Renji, Sajin Komamura, Toshiro Hitsugaya and more, but no one was more emblematic of this change than Captain Byakuya Kuchiki.