I have bad memory of beating the game, but what exactly is the relationship between Ujiyasu and Kai? I was certain that when you beat the nation in the main story, Kai calls Ujiyasu "dad", but when I look at the info in Samurai Warriors, she's simply the retainer. Maybe I got it mixed up and assumed Kai meant dad if she called Ujiyasu "old man" (because she does do that in Samurai Warriors with a slip of the tongue.
From what I understand, they were changed to being parent and child in this game because, in the original franchise, Kai simply has a crush on Ujiyasu, and they didn't want to freak people out with that big of an age difference so they made it more family-friendly for this game.
I thought they were Uncle and Niece?
When you evolve Ujiyasu, his warrior skill becomes "Father-Figure". That could contribute to some confusion as well.