Rukia Kuchiki vs. Aaroniero Arruruerie

 is a battle taking place during the Invasion of Hueco Mundo between Rukia Kuchiki and Espada #9. Aaroniero Arruruerie, who spends much of the fight masquerading as Rukia's deceased mentor, Kaien Shiba.

Prelude
After Rukia enters Las Noches alongside her friends, they reach a crossroads with five different exits and strong Reiatsu emanating from the paths ahead. At Rukia's behest, the five of them split up and each take a different exit. Some time later, following Nnoitra Gilga's defeat of Yasutora Sado, Rukia runs up a set of stairs in an attempt to reach Sado before his Reiatsu completely fades, only to stop in astonishment upon exiting the corridor to find herself underneath a blue sky on a bridge leading to a large building outside. With Rukia wondering how this is possible, Aaroniero Arruruerie appears behind her and offers to explain.

As Rukia whirls around, Aaroniero moves past her with Sonído to the front door of the building at the other end of the bridge, where he encourages Rukia to come inside with him. Once Rukia rushes after him and enters the dimly-lit room, Aaroniero expresses relief at finally being able to relax and apologetically explains that he has never been fond of sunlight, which cannot penetrate this room, before identifying himself as Espada #9. and taking off his mask to reveal that he has the face of Kaien Shiba, to Rukia's utter shock.

Rukia frantically asks Aaroniero if this is really Kaien and begins to tell herself that it cannot possibly be true since she killed Kaien years ago, but a smiling Aaroniero notes that it has been a long time and tells Rukia that she looks well. Jumping down from the pillar he was standing on, Aaroniero complains about how hard his clothes making moving around and becomes annoyed as he tells Rukia to stop staring at him since she should be happy to see everyone's favorite lieutenant alive and well. With Aaroniero requesting that she say something so he is not left standing here like a buffoon, a tearful Rukia realizes it really is him and recognizes the warmth he exudes like she has just put her hand into sunlight.

Pleased at Rukia finally believing him, Aaroniero pulls out two sitting pads and tosses Rukia one so he can sit down with her and explain the situation. Shortly afterward, Aaroniero details how his body that had begun decaying soon after Rukia killed him disintegrated in the Shiba Clan residence, which transported him to Hueco Mundo because Metastacia had been designed by Sōsuke Aizen to reconstitute there whenever it was killed, and that merging with Kaien caused it to take his form upon reconstituting as expected by Aizen, only for Kaien's mind to end up controlling the body instead because his iron will had not been factored in.

Startled by this, Rukia demands to know why Aaroniero is joking around on such a serious topic, and when Aaroniero claims that he thought Rukia looked sad and wanted to lighten the mood, Rukia clarifies that this is her serious face, which he accepts as he concludes that he took a position as an Espada and waited for a chance at Aizen following this. Bringing up how it was sunny outside, Aaroniero reveals that Aizen created a blue sky inside the dome of Las Noches and can see everything the light touches, to Rukia's shock, prior to noting that he is glad Rukia is the one who found him since she is the only one who can execute his plan. When Aaroniero asks her to come with him because they cannot talk here, Rukia immediately runs over to follow him, only to suddenly find Kaien's Zanpakutō pointed directly at her face and approaching her right eye.

Battle
With a shocked Rukia barely managing to evade this at the last second and leaping away with a large cut on her cheek, Aaroniero turns to look at her and casually observes that she has improved. Blood drips from a stunned Rukia's wounded cheek while Aaroniero notes that she looks confused and holds out Kaien's Zanpakutō as he admits to being impressed over Rukia surviving an attack that would have killed her when he last knew her, which he describes as a subordinate's progress being a superior officer's joy. When Rukia nervously wonders if this is all true, Aaroniero affirms that it is and refutes the idea that he is an impostor because he looks like himself and hopes that Rukia has not forgotten how she killed Kaien, which causes Rukia to remembers impaling a possessed Kaien with her Zanpakutō.

Aaroniero assumes that she now understands why he is trying to kill her, asserts that the time has come for him to repay the favor, and asks Rukia if she is ready to pay with her life for killing Kaien. In turn, Rukia confirms this, but clarifies that she cannot do so right now and describes how the guilt from her killing Kaien has not gone away no matter how many years pass before promising that Aaroniero can kill her if doing so will bring him peace. However, Rukia informs Aaroniero that she has come to Hueco Mundo to save her friend Orihime Inoue and will not let him kill her until the latter is safe, no matter how bad she feels about Kaien's death.

After a moment's pause, Aaroniero throws up his hands and apologizes for such a bad joke, leaving Rukia speechless while he elaborates that he kidding and does not actually want to kill Rukia even if he wants her to pay for what she has done. Walking over to a kneeling Rukia, Aaroniero gently places his hand on her head and reveals that there is something she can do for him if she truly wishes to atone for what she did. With Rukia picking up on him wanting her to do something, Aaroniero cheerfully asks her to bring him the heads of all her friends. As Rukia stares down in numb disbelief, Aaroniero states that it would be easy and points out how Rukia knowing all their strengths and weaknesses means that she would simply have to take them by surprise, which would let him forgive Rukia for killing Kaien.

When Rukia remains silent, Aaroniero wonders what is wrong, and in response to Rukia flatly observing that he is not joking, Aaroniero confirms this, leading Rukia to assert that asking her to kill her friends could never be a joke and grab Aaroniero's hand prior to angrily rebuking him for shaming Kaien's memory, to Aaroniero's surprise. Pushing Aaroniero's hand away and repelling him, Rukia emphatically declares that he is not Kaien because the real Kaien, as a man of honor whom everyone in the Thirteenth Division loved and respected, would never ask her to betray her friends. Before Aaroniero can claim to be Kaien again, a furious Rukia demands that he never say Kaien's name again and releases her Shikai, Sode no Shirayuki, which leads a grinning Kaien to hold out Kaien's Zanpakutō as well.

Moments later, Aaroniero springs off a platform with his hand just before a wave of ice surges over the area where he had been located, which leads him to land on the ground below and mockingly inquire if this is all Rukia has got. As a panting and bloodied Rukia stands on the platform above him with several more wounds sustained, Aaroniero brings up Rukia's Some no mai, Tsukishiro and Tsugi no mai, Hakuren techniques to remind Rukia that he knows them very well because he is the person she invented and practiced them with. Aaroniero wonders if Rukia really believes that they will work against him, and when Rukia simply levels Sode no Shirayuki in response, Aaroniero assumes that she has either forgotten who he is or has nothing better to muster and twirls his Zanpakutō around to release Kaien's Shikai, Nejibana, transforming it into a long trident that releases a wave of water while he warns Rukia that the battle is over now.

Aaroniero levels Nejibana at a stunned Rukia and tells her to raise her sword before lunging forward to attack, which forces Rukia to frantically block the tip of the trident inches from her head with Sode no Shirayuki. Upon seeing Rukia attempting to reposition herself, Aaroniero rapidly twirls Nejibana in his hand and strikes it into the ground underneath Rukia to rip it out upward, and with Rukia sent flying into the air on a chunk of rock by this, Aaroniero twirls Nejibana once more along the ground to bring it crashing down onto her. When she blocks this as well with one hand on the blade of Sode no Shirayuki, Rukia sees a torrent of water massing above Nejibana and moves backward just before Aaroniero brings it crashing down onto the ground in front of him, destroying a large portion of the floor in the process.

As she lands behind Aaroniero, Rukia mentally observes that the unorthodox style, high-held weapon, style of spinning, dance-like technique, and method of trapping the enemy between the trident and massive waves of water are all Kaien's moves, which her eyes and body know even if her heart does not want to believe it. However, Rukia continues to clash with Aaroniero and internally asserts that while Kaien's body is here, the Kaien she is fighting is not the kind lieutenant who taught her how to fight and helped her find her place in the world that she remembers. Suddenly, Aaroniero disarms Rukia of Sode no Shirayuki with a flick of Nejibana and tells her that it is over, but Rukia holds out her hand in front of a startled Aaroniero's face and fires '''Hadō #33. Sōkatsui''', which Aaroniero dodges by moving his upper body to the side at the last second, leaving the spell to blast through the wall behind him.

Though Aaroniero praises Rukia's skill and strength in using Eishōhaki, he stops in surprise upon seeing the hole left in the wall behind him and quickly moves several meters away to a high platform with Shunpo, which prompts Rukia to wonder why he is fleeing. Recalling Aaroniero asking her to come inside when they first met here and how that did not make sense, Rukia acknowledges that he is not the real Kaien and that his words are all lies, but questions why he only removed his mask after going indoors and decides her theory is worth confirming prior to restraining Aaroniero's arm with '''Bakudō #4. Hainawa'''.

Initially assuming that Rukia is trying to buy herself time to escape, Aaroniero sees Rukia reciting an incantation for two different spells and realizes that she is performing a Nijū Eishō just before Rukia restrains his entire upper body with '''Bakudō #61. Rikujōkōrō, to his chagrin, and unleashes Hadō #73. Sōren Sōkatsui''', a massive blast of energy that destroys a large chunk of the wall directly behind Aaroniero to let in the sunlight, causing Aaroniero to scream as his flesh falls off to reveal a large glass capsule containing fluid and two spherical Hollow heads floating inside. With Rukia staring in bewilderment and wondering what this is, Aaroniero expresses annoyance at his disguise coming off and introduces himself once more. Having had his true head exposed by sunlight, Aaroniero responds to a stunned Rukia wondering what he is by expressing irritation at having to repeat his introduction and warning Rukia to not say anything about his appearance, since he has heard it all before.

Aaroniero turns his attention to the '''Bakudō #61. Rikujōkōrō restraining him, which he shatters, and comments on it being 6th Division Captain Byakuya Kuchiki's specialty, to Rukia's shock, before admitting that there is nothing in his memory of Rukia being able to use such advanced Kidō techniques and calling her foolish because she might have wounded him had she aimed her Hadō #73. Sōren Sōkatsui''' at him. However, Rukia counters that this was of secondary importance compared to her need to learn Aaroniero's true identity and her suspicion that sunlight would neutralize his ability to mask himself, and as she pulls Sode no Shirayuki out of the ground it had embedded itself in, Rukia assumes a battle stance and concludes that she can now cut down Aaroniero without hesitation.

In response, Aaroniero muses on Rukia believing she can defeat him and reappears behind her with Sonído while clarifying that while he cannot use his powers in the sunlight, he can use them without constraint in the shadows, which he demonstrates by reforming Kaien's head around his capsule. With Rukia left speechless by this, Aaroniero complains about the surveillance method employed by Aizen involving the construction of a sun in a dark world like Hueco Mundo when there are beings like himself who can only use their powers in darkness. After asking Rukia if she believes he has the power to transform, which she nervously affirms, Aaroniero criticizes her for not knowing better, to her confusion, and informs her that he is the only Gillian among the Espada.

When Rukia bluntly wonders why this matters, Aaroniero elaborates that Gillians do not possess the abilities necessary to be Espada and, after noting that he is Espada #9. despite this, informs Rukia that he was able to join their ranks because he is the only Arrancar with unlimited evolutionary potential, which allowed him to gain Metastacia's Spirit Body Fusion ability by consuming the latter's rotting body when it returned to Hueco Mundo. With Rukia picking up on his mention of consuming Metastacia, Aaroniero pulls the glove off his left hand to reveal his Zanpakutō, a circular mouth with several fleshy tendrils taking the place of his left hand, and reveals that it lets him absorb the Reiatsu and powers of dead Hollows by consuming them, which is what let him take Kaien's appearance by consuming Metastacia after the latter possessed Kaien. As Rukia realizes what this means, Aaroniero confirms that the recognition she felt was not an illusion because he is using Kaien's real spiritual body, containing all of his thoughts and experiences exactly as they happened.

Seeing Rukia rendered speechless by the fact that he only had to act upon those memories, Aaroniero declares that it is over and decides to show her an Espada's release as a parting gift before activating his Resurrección, Glotonería, creating a massive, blob-like mass beneath his main body with dozens of tentacles and a massive face akin to a blobfish. As Rukia stares in shock at this, Aaroniero calls the terror on her face beautiful and proclaims that it will be multiplied by the physical manifestation of all the Hollows he has ever consumed.

With Aaroniero warning Rukia that the release of an Espada's power is far above that of a normal Arrancar because Glotonería can employ the powers of all his victims simultaneously and proclaiming that he has consumed 33,650 Hollows, which makes fighting against him akin to battling over 30,000 Hollows at once, Rukia leaps forward to attack him as she mentally apologizes to Kaien for causing his body to fall into the hands of the Hollows by killing him that night to save her own life, even though she had kept telling herself she had saved Kaien by recovering his body, which has ended up not being true. Struck hard by one of Aaroniero's tentacles, Rukia admits that she no longer has the strength to swing her sword or strike down Kaien's body and bids him farewell just before a thrust of Nejibana snaps the blade of Sode no Shirayuki in two and impales Rukia, who is held aloft by a maniacally laughing Aaroniero.

Slipping into unconsciousness, Rukia remembers the first day she trained with Kaien, where the latter explained his theory on hearts forming between people who are close to each other and being left with others when a person dies, which is why Rukia must not die alone. Shortly afterward, Rukia regains consciousness and grips the handle of Nejibana, leading a surprised Aaroniero to note that he is not sure if Rukia's survival is impressive or pitiful since she will never be able to kill him no matter how long she clings to life while ice begins to encrust Nejibana outward from her left hand, which Aaroniero narrows his eyes at.

Rukia states that she has remembered it as she lifts the broken Sode no Shirayuki to Aaroniero's face with her trembling right hand, and though Aaroniero mocks her for pretending to fight with a broken sword instead of dropping it, Rukia performs San no mai, Shirafune, forming an icy blade outward from Sode no Shirayuki that pierces through a stunned Aaroniero's head and the capsule inside. After elaborating that she remembered where Kaien's heart is, Rukia tells Aaroniero that he only has his spiritual body, while his heart remains with her, before bidding him farewell and shattering the capsule by slashing downward.

Aftermath
As the contents of Aaroniero's capsule spill out, as Glotonería collapses in on itself in a large wave of fluid and Rukia falls to the ground alongside Aaroniero's body, with one of his heads screaming in agony over its inability to breath and demanding to know why Aizen has not saved it while the other simply curses and cracks in two from being impaled by Sode no Shirayuki. Nearby, a struggling Rukia attempts to crawl away with pieces of Nejibana still lodged in her abdomen and remembers Kaien's instructions for her to not die alone as she remembers all of her friends and family.

Upon thinking of Orihime, Rukia mentally assures her that she knows the loneliness of captivity, the joy of being rescued by a friend, and the horror of seeing a comrade fall, which leads her to promise to come for Orihime just before collapsing.