The best way to describe the Sui-No-Sato is Shinto with Onmyodo elements as well. The way of Kami. And when you do, you need to throw you any western view point you have of religion (like how Kami doesn't actually mean God). The entire area of the Ruby Sea, Sui No Sato ESPECIALLY is based on the folktale of Urashima Tarō. The Sui no Sato are specifically, in this case based on Ryujin's servants to his dragon palace. (As you see the Rean as body guards in Shisui of the Violet Tides.) Also Shinto = Shrine, Buddism = Temple. And interesting thing to note as Shisui has a temple in its dungeon. But I am unsure if the localization team changed it to temple because people mix this up all the time.

The inspiration of Onmyoji is also here as well. Trying to find historical information on actual Onmyodo is hard if you don't know Japanese because its extremely fantastical in anime, games and other media. But Onmyoji played critical parts in the Hein royal court. And we can see that inspiration in how the Rean might interact with the dragon palace along with their dress. The dress in Suikan (or was it Kariginu? I don't remember) This is clothing from the Hein era and heavily associated with Onmyoji as back then Onmyoji were most prevalent combated to other time periods. Actual teachings of Onmyoji in the modern sense are pretty much impossible to learn and very secretive. So That is very fitting for the Sui no Sato.

I think once you learn about Shinto and Onmyodo and combine it with the Lore of the game's region and you will have a solid bases on how Sui No Sato works and making your character.

Someone already mentioned White Mage and Samurai for jobs, but Astro will also work. As Kugane has 'onmyoji that uses geomancy' and that just Astro really.

These are analogous, and not actually saying White Mages in Kugane are Priests. But these classes work for the lore related occupation of your character.
Shinto Priest = I would use White mage
Onmoyji = Astro/Geomancy

Its late for me while I write this so any question I can try to answer.