The Kingmaker served as Sia’s representative, tasked with the responsibility of evaluating the Race Sovereigns and bestowing legitimacy through crowning them. That was probably what Artasia meant when she said that he was the inspector.
It was similar to how ancient China’s imperial censors kept tabs on the officials of the imperial court to ensure that they didn’t overstep their boundaries, just that the power accorded to the Kingmaker was much greater and more concentrated. His ability to govern the Race Sovereigns meant that he was, in a way, second to only Sia Herself.
Back when Sia was still around, the Kingmaker’s position was unshakable due to Her mandate. That was no longer the case, but the Kingmaker remained a special existence by virtue of the contracts he had sealed with many generations of Race Sovereigns. Even the Mother Goddess and Savior would be influenced by him.
Whoever the Kingmaker sided with would be granted Sia’s legitimacy.