Soul

A soul is the essence of a being and an unexplainable force that grants sentience to those who have it. All sentient creatures have souls, or once did. When a soul is free to leave a dead body, it naturally changes planes of existence and enters the Underworld to rest in peace. 
Souls are recycled into a new life after spending millennia within the Underworld. Some history states beings strong enough to resurrect lives after finding their souls and reuniting it with their bodies. 
Souls can be put in almost anything which results in Youkai.
== Sentience ==
When a Soul is inside of a body it has sentience. When it is taken out, it is just a mindless soul. When Souls are sent to the Underworld after a person dies it no longer has the thoughts or memories of the person it used to be. So don't worry about your dad's soul suffering in hell. He won't experience it.
== Feeding ==
Some creatures are able to feed on souls to grant them passage from the Spirit World to the Physical World. But this is not permanent.
It is a special type of magic that lets someone feed on a soul and gain powers based on what it was in its life for time depending on its strength. Usually a weak soul will have lingering effects for a day or two. But a strong soul can last weeks at a time.