Exhaustive

In this world, there is a unique being. As soon as he loses consciousness, his soul will travel into anyone's body. He has no fixed identity or a specific name, he just knows how to act as he pleases and acts on instinct. (The first 16 chapters are dual story lines, telling stories about youth in the old world and gods in the new world. The two seem to have no connection, but they are both the same world.)