Ningyo Domain

Off the coast of Edo, under the waves, lies an insular hermit kingdom of the mermaid like Ningyo. A proud and haughty race, the ningyo prefer to be left to their own devices and manage themselves apart from the rest of society. What little interaction the Ningyo have with the other yokai come in the form of harbor trade and sporatic settlements inside lakes and other bodies of water in the interior of the spirit world. Very few Ningyo choose to live their domain to live amongst the other yokai.
Certain currents known exclusively to the Ningyo can carry one through thin spots and holes in the Seishin barrier, allowing them to slip into the normal world where they will occassionally harass or drown humans. generally speaking Ningyo do not like to go to the normal world and wont bother unless bothered.
The Ningyo represent the only fully seperate governing body from the Court of Demons, having their own sovreign royal family. The current royal family is Lord Umito, his wife Lady Minori, and their daughter the young Lady Kaiyo.
Ningyo culture is very ostentatious, steeped in a slow changing tradition, and heavily skeptical and resistant to outside influence. Other yokai are generally forbidden from entering the kingdom, and are often percieved as dirty and lesser. The Ningyo do trade their unique wares to the other yokai, including handcrafted sea jewelry and other ocean unique items, however are stringent on what gets brought back, barring anything that could be percieved as culturally destabilizing or impurifying. Ningyo eat a purely seafood diet of fish, squid, octopus, molluscs, seaweed, etc., never running out of the absundant underwater resources surrounding them.
To protect this domain from the encroachments of organized opposition from other yokai, the Ningyo have one of the offspring of the legendary Yamata no Orochi, named Orochitaro, guard and patrol the water above the Ningyo kingdom. In addition to this frightening and insurmountable beast, other waterbourne yokai like the massive shadowy Umi Bozu, catfish so large that they cause earthquakes with their tails, colossal salamander, or the siren like shapeshifting Sazae Oni that lure seagoers to their deaths. The most ubiquitous other denizen of the oceans are the enslaved Kappa. These turtle-like creatures are not as intelligent or sophisticated as the Ningyo, and thus find themselves doing the hard work the Ningyo don't want to. The Kappa lack the means or will to change this state of affairs so it continues ad infinitum. Kappa will often serve as fodder workers and soldiers under their lordly Ningyo overclass.