Basara by Yumi Tamura. A girl in post-apocalyptic Japan poses as her brother, the 'Chosen One', in order to lead her people to overthrow the king who killed him. I read it about three years ago and it's still one of the top 3 manga I've ever read. The art is gorgeous too, very distinctive. There's an anime too but it's only 13 epiodes and covers very litte of the manga, I'd definitely recommend you read it instead.