In the story of Bleach, Captain Byakuya Kuchiki served as an example of what a Soul Reaper commander is supposed to be: powerful, high-class, disciplined, wise and merciless to his enemies. During the Soul Society arc, Byakuya unveiled his deadly shikai, Senbonzakura, and released the full power of his bankai during that story arc's climax.
Bankai is a rare and powerful ability. According to Byakuya, every bankai wielder will have his or her name etched into the annals of Soul Reaper history. While many more bankai have been revealed since Senbonzakura Kageyoshi, Byakuya's own bankai still stands above the rest.
Bleach's Shikai and Bankai Often Reflect User Personality
Many shikai and bankai reflect the personality of their users, though some of these reflections are closer than others. For example, Captain Mayuri Kurotsuchi's bankai is a monstrous creature that wields the power of mad science, while the assassin Captain Soi Fon oddly has a cannon-like bankai. Meanwhile, Captain Kuchiki's bankai, Senbonzakura Kageyoshi, fits his personality perfectly, and that's more significant than it sounds.
Byakuya was the first Captain ever seen in the Bleach series, setting an example of what a Captain "should" be, a control group that other Captains may be compared to. It only makes sense, then, that his bankai is exciting but not terribly exotic when compared to that of Captain Kaname Tosen or Captain Kurotsuchi. It, too, is a control group -- but that doesn't make it boring. Byakuya is the head of the Kuchiki family, and a lot is expected of him. He indulges in calligraphy, walking in beautiful gardens, Soul Society politics, fancy clothes and more, but he is a warrior under that glitzy exterior. Byakuya is expected to both immerse himself in and defend the high culture of the Soul Society, being the blossom and the blade in one package.
Byakuya's shikai and bankai both embody this, as the zanpakuto will dissolve its blade into countless sharp pieces that float around. They catch the light in such a way that they resemble pink cherry blossoms floating in the wind. However, these are not gentle petals -- they are ready to slice and dice, representing and bloodily defending the Soul Society's beauty at the same time. This bankai is a wonder to the eyes and a devastating foe for Byakuya's enemies, just like the man himself.
How Byakuya's Bankai Keeps Growing In Spirit
The basic powers and abilities of Senbonzakura Kageyoshi don't change as Bleach progresses, but Byakuya's own perspective on his zanpakuto changes, so in a way, Senbonzakura evolves to match its owner. This further reinforces the notion of a Soul Reaper and their bankai being trusting partners, rather than master and tool. Byakuya's use of Senbonzakura Kageyoshi actually reflects his own personality and attitude at the time.
In the Soul Society arc, Byakuya was arrogant and conceited, looking down on his foes as unworthy. He only reluctantly used shikai against them, since he didn't want to acknowledge his enemies as being worth the effort. During Byakuya's final duel against Ichigo, he kept holding back his bankai, and even then, he used a restrained version, unwilling to flatter Ichigo with his full power right away. This gave Ichigo, the underdog, a fighting chance against such a powerful foe. In the end, Ichigo and Byakuya achieved a draw.
How Byakuya Got His Bankai Back In Bleach
Later, during the Hueco Mundo arc, Byakuya was a changed man. He had since given up his stubborn views on honor and the law, and instead fully embraced his role as a responsible and protective big brother to Rukia Kuchiki at last. Now Rukia is the pride and joy of Byakuya's life, not ancient laws and decorum, and this fact reflected in Byakuya's duel against Zommari Rureaux, the 7th Espada. Once Byakuya finished figuring out his opponent's abilities, Byakuya used bankai right away and utterly crushed his foe without mercy. He explained that he was slaying Zommari not out of Soul Reaper duty, but to protect Rukia. Byakuya's full might represented his fierce determination to protect Rukia from this Espada, with no arrogant pride to get in the way.
Byakuya learned to appreciate the use of his shikai and bankai all over again after the Sternritter attacked. Sternritter "F" As Nodt had stolen Senbonzakura Kageyoshi until he discarded it, allowing Byakuya to reclaim it. After losing his bankai and nearly losing his life, Byakuya got the chance to step back and re-examine his zanpakuto with an artist's eye. Byakuya and his sword are now closer than ever, and never again will he take his shikai or bankai for granted. Few other Soul Reapers have such a perspective on their own power.