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The story is about a world in which humans and monsters co-exist; but since many years ago both races have been at war, aiming for the complete extermination of each other.
Among the monsters arose an extremely powerful one, the Demon Queen; the leader of the monster race, managing to slay half of humanity all by herself.
The surviving humans elected a Hero whom would have the duty to kill the Demon Queen and release humanity from her oppressive rule.
But the Hero, Exa, questions his mission, for he anguishes upon the fact that all monsters are living creatures, the same as humans, and does not wish to kill any of them. Sheila, the Demon Queen, becomes interested in the Hero's principles and decides to hide her identity and join his side. Eventually however, she realizes that she has fallen in love with Exa, whom despite his wish for humans & monsters co-exist peacefully, has vowed to kill the Demon Queen himself.
-- from Wikipedia
The demon queen subdued enormous and powerful monsters and slaughtered over half of the world's population. Surviving members of the human race elected a hero to slay the demon queen, beseeching release from her oppressive rule. However, he questions his mission to eradicate the monsters: in his opinion, they too are conscious and alive, and so he anguishes over that fact. Sheila the Demon Queen became interested in him and joined his journey, hiding her identity and with the plan to kill him after she has gained his trust. Eventually, she realizes that she fell in love with the hero ...
-- from Manga-Updates
Note #1 - Completed at 9 volumes as of the conclusion of Square-Enix's GanGan Powered magazine imprint in 2009. The story continues in the sequel, "Superior †" (Superior Cross), which is serialized in Monthly G-Fantasy.
Note #2 - Translations are almost completed on OneManga: http://www.onemanga.com/Superior/