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From Breaking Down the Supernatural Ending of The East Palace:

In Korean folklore, ghosts are known as gwi-shin. In the world of The East Palace, gwi-mae are the ghostly spirits of people who died with lingering remorse that keep them tied to the mortal world. Won-gwi are the spirits of those who died with great resentment, and that are looking for revenge. An ak-gwi, which Gu-cheon fears becoming, is a spirit that has killed people and that has grown more corrupt with the act. 

The East Palace also incorporates ggeomeoksali, spirits that drain living things of their “Yang” energy. Gu-cheon, who lacks Yang, is fearful of ggeomeoksali, but accidentally befriends one during his time in the palace. A small, bumbling gremlin in the realm of mortals, the ggeomeoksali grows big and fearsome in the spirit world.