Manga also known as Her Highness is a Novice, by Kosumi Fujiko. Serialized in the Asuka magazine.
Summary: Shuka, princess of the kingdom of Kouka, was raised in a very sheltered environment in an inner palace. As her fifteenth birthday approaches, her father and brothers die one by one because of a sudden epidemic, and the kingdom has to find a new king (or queen)…! Supported by Zen, a military officer who serves as her retainer, Shuka must face the challenge of a role she knows little about…
Genres: Comedy, Fantasy, Romance, Shoujo