Bleach is a shonen action story where the hero, Ichigo Kurosaki, embarks on a semi-isekai journey to realms such as the Soul Society and the dead sands of Hueco Mundo, meeting many new allies and enemies along the way. It's a big spiritual world out there, and many Soul Reapers are more than they seem to Ichigo's eyes. This includes Captain Retsu Unohana of Squad 4.
Unohana is quite pleasant on the outside, being a gentle, motherly Captain who's an expert with healing kido. She even helped Ichigo and his friends recover from their wounds near the end of the Soul Society arc with her manta-like shikai. But Unohana has a very dark past that Ichigo never got to see.
Yachiru Unohana: A Wanted Criminal Turned Soul Reaper
More than a thousand years ago, the Rukongai district of the Soul Society was home to a powerful criminal known as Yachiru Unohana. She was a master of the sword, but too powerful for her own good -- she felt despair at not having a worthy opponent to fight. So she wandered the lands seeking a way to satisfy her sword, until a certain Shigekuni Yamamoto recruited her to help him form a brand new organization, the Gotei 13. Powerful as she was, Yachiru was assigned to command Squad 11, the most brutal of them all. Captain Unohana was little more than a thug in uniform, the same as the other Soul Reapers of the time, but she still couldn't find a worthy opponent -- until meeting a certain child.
In the poorest district of Rukongai, Captain Unohana came across a boy who had seized a fallen Soul Reaper's zanpakuto, and had incredible strength to rival her own. They did battle, and Yachiru suffered a wound to her throat that would leave a permanent scar. Yachiru, delighted at finding her true equal at last, hoped the boy would someday surpass her and become a true Kenpachi. She herself had the title of the first Kenpachi at the time, but that wouldn't last forever.
Sometime after, Yachiru changed her name to Retsu and gave up her warlike ways, learning medical kido from Squad 0. She also got reassigned to Squad 4 and has served as its Captain for many centuries. By Ichigo's time in Bleach, Unohana was a longtime veteran Soul Reaper. However, Retsu never forgot about that scar on her throat, and even rearranged her hair to hide it. Her warrior side was buried, but not gone.
Captain Retsu Unohana Fights One Last Battle
Captain Unohana played a minor role in Bleach's Soul Society arc, suspecting that Captain Aizen's corpse was a fake. She also made a brief appearance in the Hueco Mundo arc, healing Chad and an Arrancar before helping Ichigo make his way to the fake Karakura Town. Captain Unohana's finest moment came sometime later, when the deadly Wandenreich attacked and killed Captain-General Yamamoto. By now her throat scar was itching again, and she and her old protege, the boy who was now Captain Kenpachi Zaraki, faced her once again. This rematch was centuries in the making, and only one of them could survive.
This was Yachiru/Retsu Unohana's finest moment in Bleach, when she reignited her bloodlust and crossed swords with the new Kenpachi with all her might. She didn't just stand up to Kenpachi -- she pushed him to the limit, even using her fearsome bankai, Minazuki, to inflict true despair on her protege. And, as hoped, Kenpachi found the strength to overcome these challenges and strike Unohana down to unlock his true potential. She died happy at the hands of the ferocious boy she found centuries ago, amused at how Kenpachi cried out in despair over her fate.
In the end, Unohana could never find true happiness in battle or even as a healer. Instead, she took satisfaction in raising Kenpachi as the ultimate warrior, a proud mentor who wished her student well as she died by his sword. When someone is too strong for their own good, they find true happiness through others instead, and that's what Unohana did in Bleach. In fact, Kenpachi repeated what she did for him by adopting a lost child as his own and training her. In this case, Kenpachi "adopted" a young girl named Yachiru Kusajishi, and perhaps one day, he will do for little Yachiru what the elder Yachiru once did for him. It would be a fine legacy for Squad 11's first Captain.
