The Kaiju (界樹 Kaiju, literal meaning "World Tree" or "Tree of the Worlds") are gigantic magical trees found on the Kaishin. These trees grow magical apple-shaped fruits, which in turn sprout a race of deities known as Shinjin. Some Shinjin are selected to possibly become Kai or, if born from a golden fruit, automatically become Supreme Kai. Evil Shinjin born from the fruit are cast into the Demon Realm ruled by Makaioshin in order to be separated from civilizations, and they become Makaio.
It is also possible for Shinjin who have attained the position of Kai to be promoted to the position of Supreme Kai as shown by Chronoa after she raised the divine bird Tokitoki into an adult after saving the bird from Demigra after the wizard abused his position as one of Tokitoki's caretakers. For raising Tokitoki, Chronoa was promoted to Supreme Kai of Time, thus it is possible for a Shinjin to become a Supreme Kai even if they are not born from a golden fruit as Chronoa would have automatically obtained the position of Supreme Kai had she been born from one.