Retsu
Unohana

Retsu Unohana spends most of the series as the gentle captain of the 4th Division, the healer whose braided hair hangs down her front and whose soft smile makes Kenpachi-sized men behave in her infirmary. The Gotei 13's worst-kept secret is how they all defer to her, and the series eventually explains it: the braid covers a scar, and the name Retsu covers Yachiru Unohana, the first Kenpachi, the most diabolical criminal Soul Society ever produced and the founder of the 11th Division's way of war.

Her history with Zaraki is the arc the reveal exists for. Long before the story, a nameless boy in the Zaraki district gave the greatest swordswoman alive the only real fight of her life, and both of them unconsciously crippled themselves afterward so the joy of that day could never be surpassed. When the Thousand-Year Blood War demands a Kenpachi at full power, Yamamoto's successor orders her to fix what she broke.

The fix is the series' darkest training sequence. In the underground prison she fights Zaraki to the death again and again, healing him each time he falls, peeling away the seals his mind had placed on his own strength. When he finally surpasses her, she dies on his blade, satisfied, making him say her name. The healer the audience met in episode 1 was the apology; the sword was always the truth.