From Mangascreener:
The main character is Leonora, the daughter of an impoverished noble of Padua. She is a beautiful silver-haired, emerald-green-eyed girl and very good at fighting with the sword.
One day, she is forced to marry an arrogant young noble, but she discovers on the day of their wedding that this man intends to use her to become king of Italy because she is involved in an ancient Italian legend: Leonara is the girl shown on “Kakan no Madonna” (Madonna of the Flower Crown), a famous painting drawn by Leonardo DaVinci. There exists a legend that tells whoever gets the legendary sword “Emerald Lion” will unite and rule over Italy. And the secret hiding-place of the sword can be found with “Kakan no Madonna”–that is, Leonara.
In order to find out the truth of the portrait’s secret related her and to hide her identity as Kakan no Madonna, Leonara disguises herself as a black-haired boy, Leo, and sets out to find DaVinci. On the way she meets Falco, the blond-haired handsome king of Naples, who is unaware that she is a girl, and becomes his servant.
When she finally finds DaVinci, she hears with surprise that she – the girl portrayed in Kakan no Madonna – is destined to possess the legendary Emerald Lion and therefore to become queen of Italy.