The Kaiju (界樹 Kaiju, lit. "Worlds' Trees") are gigantic magical trees found on the World Core.
These trees grow magical apple-shaped fruits, which in turn sprout a race of deities known as Core People. Some Core People are selected to possibly become Kai or, if born from a golden fruit, automatically become Supreme Kai. Evil Core People born from the fruit are cast into the Demon Realm ruled by Demon Supreme Kai in order to be separated from civilizations and they become Demon Kai.
It is also possible for Core People 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 Core Person 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.