As you can also tell from these beautiful, far from Japanese, looks, Yuna is not pure-blood Japanese. She's a quarter-blood with a westerner as grandmother.
生粋のお嬢様だからな。残念ながら悪戯とかじゃなくてあれで素だ。
She's pure bred blue-blood you see. Unfortunately that's no sort of put-on but her natural self.