Hoshi Furu Oukoku no Nina

Nina had a rough start to life, stealing to survive—and eventually being sold into slavery by her own brother. But to her surprise, her captor, Prince Azure, ordained that she would live the life of a princess...specifically, that of the recently deceased princess-priestess, Alisha. But despite her changing fortune, Nina won't give up her old life without a fight...and Azure might just be the one to finally match her wits. But how much can she trust Azure? And can she stop the feelings budding in her heart, knowing she must eventually marry another...?
(Source: Kodansha USA)
Note: Won the 46th Annual Kodansha Manga Award for Best Shoujo Manga.