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Umi-bouzu is an ayakashi who is confined to the darkest corner of the ocean.
He has two appearances though both forms he has big wide eyes and misty purple coloring. His bigger form he has bright glowing red eyes and no arms or legs.
Every 100 years, the umi-bouzu leaves its confinement to attend a ceremony run by Orio-ya.
Not much was revealed about who the umi-bouzu was, or why he needs to be satisfied every 100 years. It was only understood that if the ceremony failed, the southern land would be subjected to natural disaster and calamity. So umi-bouzu was feared and stigmatised in a way, and was thought to bring great misfortune on all who looked upon him.
However, in an encounter with the umi-bouzu, Aoi learns that umi-bouzu is in fact a small child (in his true form). Through a vision given to Aoi, Ginji, and Ranmaru, they learn that the umi-bouzu does not bring misfortune, but rather he protects the ocean from the impurities that seep out from the darkest corner of the ocean. Of course, a job like his is taxing, so he attends the ceremony every century to regain his strength so he can continue to keep the southern land and the ocean free from danger.
Aoi also learn that the umi-bouzu is understandably really lonely, and unfortunately – so is the ceremony. So Aoi decides to keep him company during the ceremony. Later, Ranmaru announced that henceforth the ceremony would be a warm reception of appreciation, rather than one of clinical offerings.