Retsu Unohana

Retsu Unohana (卯ノ花 烈) is one of the most important and complex characters in Bleach.

She is the former captain of the 4th Division of the Gotei 13 and serves as the head of Soul Society's medical and relief services. On the surface, she is the gentle, polite, soft-spoken "motherly" healer archetype — calm, compassionate, graceful, always smiling, and deeply dedicated to treating the wounded.

However, her true nature is far darker: she is the first Kenpachi (the original bearer of the title), the most bloodthirsty and lethally skilled swordmaster in Soul Society's history. Under her original name Yachiru Unohana, she was an unmatched master of swordsmanship and a central figure in the most savage period of Soul Reaper history. She personally trained (and scarred) a young Kenpachi Zaraki, and is widely regarded as the strongest Kenpachi before him.

Her dual identity — the serene healer and the legendary slaughterer — is one of the biggest reveals in the series, especially during the final arc.

In short: Gentle 4th Division captain and supreme healer ↔ legendary battle maniac and the original Kenpachi.