Lumina was a unique existence. It was a country where every single citizen was of the same race and had an affinity for light. That's not to say there weren't exceptions, but their own parents found and exiled those exceptions from the country. It was not strange for them to be dropped off in Estia if not killed on the spot. It became a lucrative business to take the discarded youth from Lumina and sell them to houses looking for servants, especially unique and exotic servants.
They were also unique in that the cultivation technique they practice affected their personality. This was designed on purpose. A man who called himself Wiseman Key created it. He founded Lumina however long ago and passed down a single cultivation technique.
The race who ruled Lumina belonged to had a light affinity, the 3rd most powerful and one of the rarest affinities. So, it's only natural they became Imperialists. The part about being stoic at all times came from Wiseman Key himself.
