>>96380122
Korea's monsters are, for the most part, derivative of Chinese and Japanese myths. Even their vampires are literally just jiangshi, their goblins/ogres are just slightly less angry Oni and their ghosts are literally just yurei.
They do have some cool characters, like the Jeosung Saja which is a grim reaper/Charon type character. That would work better for Wraith than KOTE, though. Their "lesser dragons" would work as a Mokolé clutch, their phoenix rip off could work as maybe a minor changeling. There are a couple of types of wandering spirits in musok (korean folk shamanic religion) that could maybe work.
In general, I would instead keep the kuei jin in Korea and simply give them a different outlook of undeath, maybe one less aligned with taoism and chinese mythology and closer to Korean afterlife narratives.
As for Japan... it's difficult. Japanese folklore is different enough to deserve its own deep dive. It would make sense for them to have their own "mini splat" within KOTE instead of being just kuei jin with a slightly different name. The problem is that almost no creature in traditional Japanese stories truly fits the profile. Their blood drinking monsters are living creatures. Yurei/Onryo and the like work better for Wraith. Yokai fit better as umbran spirits (like the umbrella kid, or anything else born from objects) and bygones (like the kappa).
I do, however, have an alternative suggestion: What if we take an obscure group of spirits from an ancient japanese text, one that barely gets a few lines, and develop it into whatever we want or need it to be?