Roselia Oukoku Monogatari: Himegimi to Utsukushii Kyuukonsha

At the age of 17, Roselia is crowned Queen of the Bara Kingdom, but she is actually an ordinary girl with natural dispositions. At a festival, she meets Hernan, a man from another country, and is fascinated by him. But he is an assassin sent from a neighboring country to kill her...! (Source: Beaglee, translated)