Otome is American
Her parents were ambassadors from America (or somewhere 'Western') who were pregnant with Otome whilst visiting Japan. On the day that Otome was born, her parents were targeted by political assassins. Her father managed to grab his newborn daughter and take her to his good friend, Mr. Tsuzuki before he was caught up with and killed. It should be noted that Otome directly says that she herself was an orphan adopted by the Tsuzuki family. Her foreign parents are the reason that Otome is so tall and busty for a Japanese woman.