yooo, backstories, why are the keyblade wielders wielding keyblades? passed down from a master that chose them? or strong-hearted-summoned-out-of-pure-will sort of deal?

They were chosen during the Keyblade Wars

Kou, however, was chosen because of his incredibly powerful soul (basically where most keyblade wielders draw strength from their comrades, Kou gets his strength from his “inner fire”).