The Supreme Kais possess godly powers and live outside the main universe macrocosm on the Sacred World of the Kai. They far outclass the regular Kai as while the lower Kai watch over the living world, the Supreme Kais watch over both the Other World and the living world.
North Kai, also known as King Kai, is the governor of the north quadrant and a big part of the series. He is the strongest out of the four Kais because his fighters showed exceptional skill despite seeming more human than the others.
Shortly after, Grand Kai holds an Other World Tournament on his planet with the prize being a lesson from himself. However the fight ends in a draw between Goku and Pikkon. Despite his position, he is not as strong as Pikkon or Goku, due to having neglected his training.
The Elder Kai is a Supreme Kai, he's just older because he was sealed in the Z-Sword for (I think) 15 generations of Supreme Kais. The major difference between Elder Kai and Supreme Kai is that Elder Kai was the Supreme Kai of a different era.
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