Kon & Haku
History, Heritage and Religion
There are two parts to the spirt, haku and kon. Haku is the part of the spirit that resides within the body. It has it's own physicality. When the body dies haku returns to the earth. The haku is what dictates the physical. With just haku, you'd be something akin to a living doll- capable of movement but not thought.
Kon, however, does not reside in the body and does not have its own physicality. When the body dies kon returns to heaven or the ether.The kon is what gives you "life," in a sense.
When kon and haku are unified, it is called kokoro. Kon and haku are interactive and independant. They should be kept in balance.
Relevance to Inuyasha
An example of a soul with only haku is Kikyo prior to her gaining a portion of Kagome's soul. Primarily it deals with the creation of Moryomaru in chapter 339 where Miroku explains kon and haku as it relates to Moryomaru's birth.