Retsu Unohana, originally Yachiru Unohana, served as a founding member of the Gotei 13 and the inaugural captain of the 11th Division, claiming the title of the first Kenpachi as Soul Society's most formidable swordsman. Before this, she stood as the realm's most diabolical criminal, a notorious and bloodthirsty killer whose early life was defined by violence. Genryūsai Shigekuni Yamamoto recruited her into the original cadre of captains. Roughly one thousand years prior to the Quincy Blood War, she fought alongside Yamamoto against Yhwach and his invading Quincy army. She established the 11th Division's core identity focused on melee combat and swordsmanship. A pivotal encounter occurred in the Rukongai districts with a young Kenpachi Zaraki. Tasked with eliminating warriors there, Unohana felt disinterested until the child Zaraki attacked her, inflicting a severe wound between her collarbones that left a permanent scar. This battle reignited her passion for combat; she recognized Zaraki's latent superiority and potential to become the true Kenpachi, noting he subconsciously sealed his power during their fight to prolong the engagement, ultimately leading to his defeat. Believing Zaraki deserved the Kenpachi title, she later relinquished command of the 11th Division. She transitioned to lead the 4th Division, succeeding Tenjirō Kirinji after his promotion to the Royal Guard. Under Kirinji's instruction, she trained extensively in Kaidō (healing Kidō)—initially to prolong battles by healing opponents, but later adopting healing as her primary role. As captain of the 4th Division, she cultivated a gentle, polite, and maternal demeanor, using honorifics in speech and prioritizing healing for all, regardless of allegiance. Her duties included collecting medicinal herbs, teaching ikebana, and overseeing medical operations. Despite this benevolent exterior, fellow captains and members of the 11th Division feared her, a remnant of her past reputation. She demonstrated sharp analytical skills, deducing Sōsuke Aizen's deception during the Soul Society arc and healing critically injured allies like Tōshirō Hitsugaya and Momo Hinamori. Her Zanpakutō, Minazuki, manifested in Shikai as a giant winged creature capable of healing injuries by enveloping patients within its stomach; she carried it slung over her shoulder or delegated it to her lieutenant, Isane Kotetsu. During the Quincy Blood War, following Yamamoto's death, her dormant violent nature resurfaced when Captain-Commander Shunsui Kyōraku ordered her to train Kenpachi Zaraki. She confronted Zaraki in Muken, the infinite prison beneath Seireitei, shedding her gentle facade—revealing her scar and wearing her hair down. She articulated that Zaraki had unconsciously restricted his power since their first battle to preserve the thrill of combat, viewing this as her sin. To force him to reclaim his full strength, she engaged him in a brutal duel, repeatedly killing and reviving him using her advanced healing techniques. Her Bankai, Minazuki, flooded the battlefield with a corrosive, blood-like substance she manipulated for offense, defense, and regeneration, showcasing her dual mastery of combat and Kaidō. After numerous near-death experiences, Zaraki unlocked his potential and delivered a fatal blow. Unohana accepted death with contentment, affirming Zaraki as the true Kenpachi and fulfilling her self-appointed role as his mentor. Her character arc concluded with this sacrifice, cementing her legacy as both Soul Society's most skilled healer and its most fearsome original warrior. She is interred in Hell after death, gaining the epithet "Death Sword".