Simply put, no, Ukitake did not become a Soul King, nor did he try to. Instead, he sacrificed himself to allow Mimihagi, the Soul King's right-hand man, to ascend to that position.
Do you remember how chaotic Soul Society was after the death of the Soul King? That's because the Soul King acts as a pillar, binding the different realms together. Without him, everything would fall apart.
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Ukitake, who always had a noble soul, decided to take action. He performed a ritual called “Kamikake,” allow Mimihagi takes over his body and essentially becomes the new Soul King (Bleach: TYBW ep 29).
The reason he was able to perform this ritual was because Ukitake suffered from a terrible lung disease as a child, and his parents, in desperation, prayed to Mimihagi.
Mimihagi answered their prayers by taking Ukitake's lungs, effectively curing him but also creating a lasting bond between them.
The Kamikake ceremony is not about Mimihagi possessing Ukitake, but about Ukitake offering himself to make Mimihagi whole.
Remember that Mimihagi is just a part of the Soul King, his right hand. By fusing with Ukitake, Mimihagi has the ability to regain his full power and become the new Soul King.
In Kamikake, a shadow resembling Mimihagi rises from Ukitake's back, reaching towards the sky. Darkness erupted from his body as Mimihagi flew up.
Upon arriving at the palace, Mimihagi encased the remaining remnants of the Soul King crystal, shocking everyone present. This action temporarily stabilized Soul Society, stopping the tremors and preventing immediate collapse.
Is Mimihagi now the Soul King?
Again, the short answer is no. Mimihagi also cannot take the position of Soul King thanks to Yhwach's intervention.
While Mimihagi's arrival at the Elemental King's Palace and subsequent merger with the Elemental King's remains offers a glimmer of hope, Yhwach is determined to prevent any attempt to derail his goal.
Mimihagi managed to encase the Soul King's shattered crystal, temporarily stopping the tremors that threatened to tear Soul Society apart.
This gives Ichigo and his allies a chance to regroup and defend Mimihagi, while Yoruichi erects a barrier around it.
They believe that Mimihagi, as the Soul King's right-hand man, is the best candidate to take over and maintain order.
However, their efforts were hindered by the sudden appearance of Uryū Ishida, now aligned with Yhwach. Uryū brutally attacks Yoruichi, breaking her barrier and leaving Mimihagi vulnerable.
With Ichigo distracted and his friends incapacitated, Yhwach seized the opportunity. He forcefully tore Mimihagi away from the rest of the Soul King, preventing it from fully integrating and assuming the Soul King mantle.
Yhwach didn't stop there. He absorbed Mimihagi's power, gaining a distinctive eye shape on his body.
This action further destabilized Soul Society, and the tremors continued with renewed intensity. The darkness that engulfed Ukitake forced him to leave his body and is now inside Yhwach.
The Soul King's battle for power is not yet over, and the fate of the Soul Society remains uncertain. Ukitake's sacrifice, while noble, ultimately proved insufficient to ensure Mimihagi's ascension.
What do you think about the whole Soul King story? Let me know them in the comments below!