Introduction
What is the main powerhouse of human body, the heart or the brain? Wrong my followers it is the soul that help power the whole human body while the organs (heart, brain, skin, eyes, stomach, etc.) help keep the physical body alive. There have been several subjects on soul from religion (Christianity, Hinduism, Catholicism, etc.) to the pop culture world (Soul Eater, One Piece, and Full-Metal Alchemist Original and Brotherhood), but scientists have disregarded the notion of soul and called it one's life forces (Futurama Season 7, Episode 4). Even if you call a soul by it's name or any other still does not disprove the source of human, animal, and plant life.
Chapter 1: Life
The soul can be wild or spirited, so how can a unstable, powerful force be contain within a mere mortal body? Once a baby is born they come with a built in barrier and soul either by reincarnation or the half of a soul of the parents. The Barrier help keep the soul in place no matter what like the soul is incased within a super-ball bouncing around the body as you move. Unfortunately the barrier is only temporary; once the body dies the barrier, like the organs, decay like a decomposable bag in the environment and release the soul into the world. A possessing soul like demons, angels, or ghosts can occupy any barrier while pushing the occupant's soul in the process. Think of the barrier as not only preventing the soul from getting loose, but the driver's wheel for the whole body. In the show Supernatural and Blue Exorcist, Angels, Demon, and Ghosts are born in different world and their original body can not operate well in the human world, so they must possess humans, animals, fungi, and, objects to even make contact between the two worlds. The minor demons are able to possess anyone with ease, but very powerful demons like Lucifer (S) and Satan (BE) need a suitable host because any other medium would just deteriorate for a short period of time. Exorcisms forces the demons out of the body's barrier which help the original owner gain control. Angels must have permission by the host to be able to possess them. Ghost applies the same rule to demons. What happen to the original owner is unknown to others maybe the person remain aware to the situation, or is unaware during the whole possession.