A Shaman's spirit partner who can assist the Shaman in many ways. Hao's ambition was to get the Spirit King as his guardian ghost. The More powerful a ghost is the more powerful a shaman can get, also if the level of power of the Shaman becomes to powerful than the spirit may evolve.
Types of Ghosts
The Types of ghosts seems to differ, as some (like Amidamaru, Bason, and Tokagero) are Human Ghosts while others like Kororo and Morphine are Nature Spirits.
Human Ghosts
The large population of ghosts in a graveyard are human ghosts who are often the choice for a Shaman's first ghost. They are the most common and easiest to find and summon, sense a Shaman is a human like the spirit once was.
Plant Ghosts
Hao developed a technique that summoned Plant spirit that was passed down through his decendits, the simplest being leaves, however Hao taught The BoZ Brother how to summon the Chimimoryo which resemble plant seeds in the anime.
Animal Ghosts
While not the most common to find, they are the most abondent, and the second most common and third easiest to summon. When a Shaman is in Spirit Posession from an animal spirit they inheirit some of the animal's characteristics. Despite looking like seeds in the anime, Chimimoryo are animal spirits.