Kaeya actually originates from Khaenri'ah, a land separate from Teyvat entirely. The two lands have a rocky history with one another, and Diluc is floored to discover that his adoptive brother hadn't told him about his true origins.
Kaeya's life has been nothing but rejection and abandonment, which explains a LOT as to why he is how he is in the game. Even in his own story quest, he doesn't trust the traveller with his info, preferring to manipulate them.
Kaeya Alberich is an adopted son to the Ragnvindr Family, the renowned wine tycoon. It has been a long time since he last called Diluc Ragnvindr "brother." Kaeya currently serves as the Cavalry Captain of the Knights of Favonius, and is trusted by Jean.
After diluc's father died, later that day, we also know that Kaeya confessed his Khaenri'ahn origins and the purpose of having been abandoned in Mondstat, therefore exposing himself as a liar and a spy. Enraged, Diluc lashed out and kicked him out of the house.
He had anticipated Diluc's anger. The brothers drew their blades, this time pointing them at each other. Kaeya felt that this was his punishment for a lifetime of lies.
Kaeya's Vision.
It was raining heavily in Mondstadt on the night that Kaeya Alberich received his Vision. Earlier that day, Crepus Ragnvindr had wielded the power of a Delusion — only to become its victim when it backfired. To set his father free from the unbearable agony, Diluc Ragnvindr took his life.
(I think that's all, I'll update it if I find any more information><)
