Parents can pass on congenital traits to their children that can be strengthened over generations, letting you promote things like intelligence and symmetrical features through arranged marriages and bad science. They start developing before they're even born. Everything has a root cause, something that the trait can be traced back to, like a childhood bully or a battle that went badly, creating characters moulded by their pasts. They might be greedy, cruel, pious, horny, perpetually drunk-if you're looking for an adjective, you'll find it.
Each of them is full of agency and ambitions and will more often than not devolve into a petulant child when they don't get their way. Crusader Kings has always been about characters instead of nations, but they've never seemed so rich and so maddeningly real before.
