In Chinese vampire lore there are a number of blood-drinking REVENANT and the word that is used to describe them collectively is k’uei, which translates to mean “deficient.” The idea of a k’uei being an undead entity stems from the belief that every person has two souls (see UNDEATH). The first soul, hun, is considered to be the superior soul. A person does not receive it until he is born; it enters into his body with his first breath. The second soul, P’O, is seen as the inferior soul. It is present in a human even when he is a fetus, but it merely exists in the body; it is what makes a person alive until he receives his superior soul. When a person dies, his P’O is supposed to leave the body. However, through a variety of methods or reasons, should the P’O not leave the body or if any amount of it stays behind, the k’uei that is created at the time of death will interact with the P’O, causing unlife to occur.

Source: Bunson, Vampire Encyclopedia, 147; Ouellette, Physics of the Buffyverse, 4; Rose, Giants, Monsters and Dragons, 424