The Shinjin are born from magical fruits that grow off of the Kaiju. The average Shinjin possesses special abilities that far surpass that of any sort of un-trained normal being. Shinjin live leisurely lives, studying various things at a school-like castle. The Shinjin lifespan is roughly 75,000 years, however those born from golden magical fruits can live for several millions of years.
Whenever one of the Kai on the Kai planets passes away, the Shinjin draw lots among themselves to choose who will take the Kai's place and become a new Kai. The Supreme Kais, however, are chosen only from among Shinjin born from a special golden fruit that is particularly rare. However it is possible for a Kai to attain the position of Supreme Kai as shown by Chronoa who became Supreme Kai of Time after she raised Tokitoki into a adult.
Additionally, Gowasu was originally training Zamasu to become a Supreme Kai and briefly made him a temporary Supreme Kai so he could use a Time Ring they could travel 1000 years into the future to observe how the Babarian had progressed as a species.
Most Supreme Kai have the same general appearance: tall and thin with white mohawks and sharply pointed ears, generally only differing in skin color and other minor details, though there are exceptions, particularly in the case of Universe Seven.
Occasionally, there are Shinjin who are born evil. Those delinquent Shinjin are cast into the Demon Realm ruled by the Makaioshin, where they live with the Makaio and are secluded from civilizations.