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Credit and Thanks for summary: Bleach Wikipedia with some modifications from me.
Born with an incurable lung disease, Ukitake is saved from death after his parents pray to a local deity of the Rukon district. That deity is Mimihagi, the right arm of the Soul King that crashed down to the Soul Society. It fuses to Ukitake's body, thus stopping his sickness from progressing, but not curing him.
Ukitake joins the Soul Reaper Academy under the tutelage of Yamamoto, Sasakibe and Unohana, and becomes close friends with his sparring partner, Shunsui. , confiding in him about Mimihagi.
Later the two mourn Kaien's passing, and Yamamoto's with Unohana, after which Ukitake bids farewell to Unohana.
Back in the present day, Ukitake reveals to the others that the Kamikake process he underwent gave all his organs (not just his lungs) to Mimihagi.
After handing Squad 13 over to Rukia, Ukitake uses his life force to hold the Soul King together...temporarily.
Meanwhile, Ichigo, Yoruichi, Chad, and Ganju try to distract Yhwach. Orihime tries but fails to reverse the Soul King's death. However, Mimihagi attaches itself to him, which was something that not even Yhwach's eyes foresaw.
In the Muken, Shunsui partially unseals Aizen and tries to make a deal with him to help save the Soul Society.