Key points
- Ukitake's severe lung disease was saved by the divine intervention of Mimihagi, the Soul King's right-hand man.
- Ukitake sacrificed his borrowed life to stabilize the kingdoms, fulfilling his destiny from the beginning of the arc.
- Despite his illness, Ukitake's immense talent as a Shinigami and his knowledge of Mimihagi's true nature reveal his selflessness.
In the third episode of BLEACH: Thousand Year Blood War, Part 3titled “The Dark Arm,” 13th Squad Captain Jūshirō Ukitake reveals exactly why he has been so ill his entire life and how his life-threatening lung condition was alleviated in part by the divine intervention of Mimihagi, a local deity venerated in Sakahone. the 76th district of Rukongai.
With Yhwach threatening the very fabric of the kingdoms by wanting to kill the Soul King, Ukitake sets in motion a ritual that will return the life he borrowed from Mimihagi, whose true identity is that of the Soul King's right-hand man . Here's a summary of Ukitake's backstory and the selflessness of his sacrifice.

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A young life claimed by illness
Ukitake almost died as a child
“Lord Mimihagi, Lord Mimihagi. Kill the disease that eats our son and devours his organs.
Lord Mimihagi, Lord Mimihagi. The divine hand from heaven…please bless us with your power…”
Despite being part of a lower-class aristocratic house in Soul Society, Ukitake lived in poverty with his parents, and his quality of life had been terribly hindered by an unnamed condition that was wearing on his lungs. When he was three years old, a young Ukitake was already on the verge of death, and his parents no longer knew what to do. No doctor could cure what ailed their son, so, in desperation, they took him to a statue of Mimihagi and prayed to the deity that he could remove his lungs, which allowed him to grow up as a relatively healthy boy and become a Shinigami. However, Mimihagi's intervention did not mean that he was cured, as he still coughed up blood from time to time, a sign of the time he had borrowed from the deity. During the possession, a young Ukitake has visions of a mysterious humanoid being with a fist-shaped head draped with a robe resembling the spiritual robe worn by Shinto and Buddhist priests, otherwise known as a joe. A dark shadow emanates from the entity, enters Ukitake's body and takes him from the inside, transforming him into a dark mirror of himself.
Despite the fact that Mimihagi saved his life, there were several times when Ukitake's physical condition was questioned, and shortly after Kaien Shiba's death, the Central 46 expressed their concern over the refusal to Ukitake to replace his lieutenant. Despite his unfortunate health situation, Ukitake has proven on occasion to be an immensely talented Shinigami who could have been the equal of his best friend, Shunsui Kyoraku. Mimihagi's intervention in Ukitake's illness is an expression of his authority over the concept of Stillness; or stagnation, in the sense that it has the ability to inhibit any change or development, such as the progression of a disease in the case of Ukitake. He grants gifts to those who give him something other than an eye because he already has one, and consumes the offered portion by becoming one with it, hence Ukitake's vision of Mimihagi in the primordial waters. In episode 3 of BLEACH: Thousand Year Blood War, Part 3Ukitake invokes divine possession and offers his body to become the Soul King's new right-hand man, who would then stabilize the swing of the realms towards each other.
Has Ukitake always known his fate?
He set the ritual in motion at the beginning of the arc
During the events of the third episode of BLEACH: Thousand Year Bloody War Part 3, we also see a brief anime-only interaction between Ukitake and Unohana shortly after Yamamoto's death, in which he tells her that he will see her again soon, which can be interpreted as a nod to the fact that they both lose their lives during the final arc and it is revealed that they were thrown into Hell during the No breath from hell one-shot due to the fact that souls with immense reishi levels cannot be safely returned to the cycle of reincarnation and must therefore be sent to Hell via a special Soul Burial.
We barely see Ukitake during this arc, and when we do, he's holed up in a mysterious-looking hut, which we now understand is a statue of Mimihagi. Given the fact that Ukitake and Kyōraku have a youthful conversation about Mimihagi and the rumors that it was actually a severed arm of the Soul King. Ukitake was always aware of his condition, and since he was Mimihagi's host, he also intuitively knew his true nature. Based on his actions, it's clear that he knew that one day he would have to give up the life he was given for the sake of Soul Society, and the truth behind his situation is one of the most heartbreaking yet inspiring developments of the final arc Of BLEACH.
