This article references information from the Bleach: Thousand-Year Blood War Arc not yet released in AnimeYachiru Unohana (aka Retsu) is a gentle, dark-haired beauty responsible for health and safety as the Captain of the 4th Division in Bleach. She does not discriminate, and has been known to go out of her way to heal all those in need, be it friend or foe. Her personality exudes a distinctly non-confrontational aura and a deep sense of duty that make Unohana a very likable character, so many fans were shocked to discover that most of her Gotei 13 colleagues are actually quite terrified of this timid, tiny creature. Upon first impressions, Retsu Unohana appears to be most similar to her subordinate, Isane Kotetsu, the soft-hearted Lieutenant of the 4th Division, than anyone else. However, when delving into Unohana's history, it appears that this Shinigami medic has more in common with the brute force that is Zaraki Kenpachi, who, by all appearances, is Unohana's polar opposite. It seems unimaginable that this delicate lady would possess such bloodlust and passion for fighting, but surprisingly, in her younger days, Unohana used to be a thrill-seeking combat junkie and the Captain of the 11th Division.
Before the Gotei 13 were established in Bleach, Unohana was a criminal mastermind with sinister intent who caused mayhem and murder. However, Yamamoto saw potential in the woman and approached her with a proposition, and, along with Jūshirō Ukitake and Shunsui Kyōraku, they founded Seireitei's defense force. Unohana established the groundwork for the 11th Division, which specializes in swordsmanship, and it was only much later that she began to dabble in the healing arts, thanks to Tenjirō Kirinji's instruction. Her bloodlust was still in full force during this time, as the Shinigami manages to find a sinister application of her newfound healing skills, with the desirable results of an extended battle. Unohana's favorite move was to heal her opponents moments before they died, for no other reason than allowing them to continue fighting, for her own selfish pleasure.
Unohana and Zaraki Kenpachi
As the leader of the 11th Division, Unohana was originally responsible for cleaning up the riff-raff from Seireitei. With her mission complete and so many unworthy opponents vanquished, the Shinigami became disinterested in swordsmanship as she struggled to renew her passion for combat. While working her way through Rukongai, Unohana finds a young man named Zaraki Kenpachi, a surprisingly adept fighter who shocks the Captain out of her depression. She realizes that the youngster has an exceptional power level, capable of one day claiming the title of the strongest Shinigami, and her faith in finding more epic battles is restored.
The Strongest Shinigami
Following her first battle with Zaraki, Unohana returned to Yamamoto's side, but transferred over to the 4th Division and focused more on her healing skills than her swordsmanship. Regardless of her new role, Unohana maintained her title of Kenpachi, which is an honor that only the strongest of the Shinigami of each generation may hold. The exact reason why Unohana was named the Kenpachi over the Captain-Commander is unconfirmed, although fans speculate that it was due to her broader range of skills.
As powerful as Yamamoto is, he relies heavily on his overwhelming spirit energy to do the work for him and is subsequently less worried about other essential qualities. The best description is from a reddit user (RueOrintier), who compared Yamamoto to a hammer and Unohana to a blade: both potentially deadly weapons, but with one requiring much skill to be effective, and the other needs only brute force. In a community dependent on the use of Zanpakutos, the blade would receive more praise than the hammer, therefore making Unohana the more respectable choice.
As Shinigami rules dictate, there may be only one Kenpachi per generation. Upon meeting the young Zaraki, Unohana had to admit to herself that she will one day have to relinquish this honor. The reason for her shift within the Gotei 13 ranks is uncertain, but Unohana may have had a greater plan underway. Zaraki was not yet a qualified Shinigami when she found him, yet his insane skills with a sword intruegued her. If the boy were to join the ranks of Soul Reapers one day, the 11th Division would be the perfect fit because fighting is their main priority. Knowing full well how quickly he could move through the ranks, Unohana possibly stepped aside to make way for her future teammate, passing on the title of Captain to the man who deserved it most.
It is only in the Bleach: Thousand-Year Blood War Arc that Unohana and Zaraki cross swords again, as she is the only Shinigami skilled enough to be a worthy opponent during Zaraki's training. The Quincy forces are approaching and, without his Bankai, Zaraki can only do so much to assist in the battle. Fans are given their first real taste of Unohana's incredible power, as she beats the almost indestructable Zaraki (who has already removed his power-surpressing eye-patch) to a pulp with ease. Jaws drop to the floor as the Shinigami healer continues her onslaught, slicing her opponent's throat. She heals the dying man and immediately continues her attack, embracing her long-lost bloodlust to its full potential.
Unohana states that Zaraki's powers increase immensely each time he returns from the brink of death, giving reason to her unexpected brutality. However, it is clear that both Unohana and Zaraki are thrilled to be facing off against each other once again as they push their skills to the limits. In a bitter-sweet victory, Unohana submits to her wounds and welcomes death, content with Zaraki's achievements and happy to finally hand over the renowned Kenpachi title.