Mimihagi

Back when Jushiro Ukitake is still a child, Mimihagi presents as a humanoid figure whose torso resembles an arm capped by a tightly closed right fist; one oversized eye peers from the knuckle side of that fist. A plain tunic covers it, and a broad feathered raiment fans out behind the closed hand.

In the modern era it has no solid form at all, surfacing only as living shadow spun from Reishi. That darkness slips between two guises: its old humanoid outline, its single eye rendered as a patch of shade, and a bare arm-and-hand shape of weightless black.

Next to Pernida, its opposite number, Mimihagi displays almost no discernible temperament. Even so, it answers the prayers of those who beseech it, as when it strips the fatal lung ailment from the young Ukitake because his parents beg on his behalf, a gesture hinting at some faint regard for how Soul Society's residents fare. Obedient to what the Soul King wills, it labors to steady the divided ruler so the Three Worlds will not perish, and it even rounds on Yhwach in a bid to repel and devour him.

More than a millennium ago, Ichibe'e Hyosube journeys to the Human World hoping to broker peace with Yhwach's Lichtreich, and as a precaution he arrives with both of the Soul King's arms, left and right alike, embedded in his own body. Once the negotiations fall apart and Yhwach calls up The Almighty, Ichibe'e turns the left arm's stare on the Quincy monarch and wields the right to catch his Sternritter's Heilig Pfeil in the air and hurl them back. Later the right arm awakens on its own, takes the name Mimihagi, and wins worship as a god along the eastern reaches of Rukongai; there it saves a three-year-old Ukitake by claiming his diseased lungs, letting him live to become a Shinigami.

During the second Wandenreich assault, Ukitake carries out the Kamikake rite, surrendering the rest of his flesh and his life so Mimihagi can substitute for the dead Soul King. Shadow bursts from him as Mimihagi ascends to the Soul King Palace and enfolds the split monarch, briefly stilling the quakes below. Yhwach rips it away and absorbs the Soul King, yet Mimihagi survives through its tie to Ukitake, holding the Three Worlds apart with its rule over stagnation for as long as its host draws breath.