Kenpachi Zaraki vs. Retsu Unohana

 is a fight taking place during the Quincy Blood War between 11th Division Captain Kenpachi Zaraki and 4th Division Captain Retsu Unohana, shortly after the Vandenreich's retreat from the Seireitei.

Prelude
After announcing his decision to the Central 46 of wanting to have two lieutenants, Shunsui Kyōraku reveals his first task on becoming Captain-Commander: instructing Kenpachi in Zanjutsu. Despite Central 46's vehement protests, Shunsui, calling in Unohana, states he will leave the teaching of Kenpachi to her, revealing her true name to be Yachiru Unohana, the first Kenpachi. After Shunsui gives them permission to use the Central Great Underground Prison's lowest level, Muken, as a stage for their training, the two captains descend and find themselves in an infinitely wide, featureless room. Exchanging a seemingly casual conversation about the reasons why they are both there and revealing both of them have scarred each other in their past fight, the former and the current Kenpachi draw their weapons in preparation for the inevitable battle before lunging at one another.

Battle
Unohana lands several direct hits on Kenpachi, leaving the latter bloodied while she stays unharmed. As Kenpachi lunges at her with a downward strike, Unohana, redirecting his blow with her left hand, cuts Kenpachi's clavicle open with her own blade. Jumping back a little, Kenpachi swings at his opponent, only for her to step onto his wrist while preparing her own downward strike. Seeing this, Kenpachi, flipping over, kicks her in the ribs, sending her towards a nearby rock wall. Recovering from his blow, Unohana, launching herself off the rock, clashes with Kenpachi as a shock wave emanates from their colliding Zanpakutō.

Praising Kenpachi for being smart enough to remove his eye-patch from the very beginning of their battle, Unohana points out this means he is already fighting at full strength. Impaling his left shoulder with a hidden knife, Unohana tries to cut Kenpachi while he is distracted, only for the latter to parry her blow. Moving back swiftly to get some distance between them, Unohana, calling him out on his lazy fighting style, claims people who fight with only one hand on their sword while doing nothing with the other cannot possibly be enjoying the fight. Countering by calling her trick with the knife as pathetic, Kenpachi declares she is no longer the woman he used to respect.

Angered by his admission of respect, Unohana, disarming Kenpachi, pins him against another rock wall, the tip of her blade in his throat. When Unohana explains she had not used her knives before because Kenpachi had not let her do so in the past, Kenpachi asks if he is grown weaker. Reminiscing about his past, he recalls how nothing he had cut or fought had given him a thrill until meeting her. Shedding a tear, Kenpachi asks if he is about to die a miserable death without ever having defeated her. Not answering, Unohana, impaling him through his throat, looks on as he began to lose consciousness.

Moments later, Kenpachi is surprised to find himself regaining his senses and feeling more alive than ever, with his sword in hand and blocking Unohana's blade. Wondering for a brief moment what just happened to him, Kenpachi, dismissing it from his mind, focuses entirely on the battle at hand. Thinking to herself, Unohana states Kenpachi will not be the one to die, for he grows stronger every time he nears death.

As the fight goes on, Kenpachi realizes he is continuing to black out, only to regain consciousness moments later. Waking up once again while airborne, he reaches out to the ground with one hand to slow his momentum before regaining his footing and going on the offensive once more. Disturbed by how he is constantly losing consciousness, despite never having done so before, Kenpachi remembers there had been one time he had passed out while fighting, which was when he and Unohana had battled previously. Also recalling their previous encounter, Unohana acknowledges back then, Kenpachi had been much stronger than her, and it was during this time he subconsciously placed limitations on his own power, afraid he would forever lose his interest for battle by killing her, his first true opponent.

Blacking out yet again, Kenpachi, waking up in time to avoid Unohana's attack, counters with one of his own, finally landing a blow on her. Noticing he reacted to her attacks on reflex rather than having to concentrate like before, he realizes every time he regains consciousness, he feels as though he is being reborn. Continuing to rain lethal blows on him, Unohana admits her aim is not to defeat Kenpachi, but to help him unlock the power he once possessed: by healing him every time he is about to die, Unohana gradually lifts the restraints which Kenpachi had subconsciously placed upon himself, bringing him closer and closer to achieving the level of strength which he had possessed when he had first fought her. As Unohana expresses her wish for him to surpass her, Kenpachi, recovering once more from her brutal assault, lands a devastating blow across her torso.

Recovering from Kenpachi's devastating slash, Unohana lands a blow on the right side of the man's face, and the two clash once more, with Unohana healing the wounds which Kenpachi had inflicted on her using her healing abilities. Mocking him for thinking she is dead, Unohana, stating he of all people should know why she had mastered the arts of Kaidō, activates her Bankai. As Kenpachi stares in shock at the raw display of power her Bankai has released, she warns him "playtime" is over. Excited at this new challenge, Kenpachi howls as he and Unohana continue their duel. As they exchange blow after blow, Kenpachi takes note of the change that has overcome him, feeling as if his skin had melted away to reveal the true self which lay underneath. He states it is as if he had been asleep this entire time, dreaming of their previous encounter over and over again, thinking their exchange was nameless and empty, but thanks to her guidance, he realized this is what true fighting feels like, and he thanks her for helping him see this.

Landing another blow on Kenpachi, Unohana, jumping to avoid his counterattack, muses over how each of them had found a way to continue enjoying battle, Kenpachi through sealing away his true strength and Unohana through learning healing techniques at the hands of Tenjirō Kirinji. She realizes all the power she had gained since their first encounter had been for the sake of this battle alone, for she had chosen him to be her successor as Kenpachi since that very same day. Noting there cannot be two Kenpachis in the same era, for one either uses their sword to kill or raise the second individual, she states Kenpachi was the only man in the world who could make her happy before being impaled by his Zanpakutō.

Aftermath
Smiling, a dying Unohana, congratulating Kenpachi, states it is all over. As Unohana Zanpakutō dropping from her failing fingers, Kenpachi, afraid she is dying on him, discards his blade. Catching her as she falls, he, pleading for her not to die, begs her to battle him one last time. Smiling as he releases her, Unohana, commenting to herself on how he is like a child, tells him not to be sad, for even though the battles he has in the future may return to being boring games to him, he has an enemy worthy of his strength, and allies who he can face as equals, and most importantly, his partner has awakened alongside him. Stating she has no more to give to him, and even the title of Kenpachi which she held for so long has left her, she tells him to rejoice in her death, for there is no better time to rejoice than when someone dies with one's purpose fulfilled. As Unohana dies, Kenpachi hears a mysterious voice, which, greeting him, comments on how he can finally hear his voice. As Kenpachi realizes this voice is coming from his Zanpakutō, the voice introduces itself to him.