"Demon Sacred" is a fantasy manga series set in a world where humans coexist with mysterious beings known as demons. These demons, referred to as "Sacreds," possess immense power and are bound to humans through contracts. The story begins with the introduction of Mona, a young girl who lives with her older sister, Rina, and their caretaker, Shinobu. Their lives take a dramatic turn when Mona encounters a Sacred named Raphael, who claims to have been waiting for her for centuries. Raphael forms a contract with Mona, granting her immense power but also tying their fates together.

As the narrative progresses, it becomes clear that Sacreds are not merely mythical creatures but are tied to a deeper, more complex history. The world is governed by an organization known as the "Sacred Management Bureau," which regulates the interactions between humans and Sacreds. The Bureau enforces strict rules to prevent the misuse of Sacreds' powers, as their abilities can be both a blessing and a curse. However, not all Sacreds are content with this arrangement, and some seek to break free from human control.

The central conflict revolves around the emergence of a powerful and enigmatic Sacred named Michael, who harbors a deep resentment toward humanity. Michael's actions threaten to disrupt the fragile balance between humans and Sacreds, leading to widespread chaos. Mona, Raphael, and their allies are drawn into this conflict as they attempt to uncover the truth behind Michael's motives and the origins of the Sacreds. Along the way, they encounter other key characters, including Kira, a skilled Sacred hunter, and Setsuna, a scientist with a personal connection to the Sacreds.

The story delves into themes of power, loyalty, and the consequences of human ambition. Mona's journey is marked by her struggle to understand her role in the unfolding events and her relationship with Raphael, who is both her protector and a source of mystery. The narrative explores the complexities of the bonds between humans and Sacreds, as well as the moral dilemmas that arise when power is wielded without restraint.

As the plot unfolds, secrets about the Sacreds' origins and their connection to humanity are gradually revealed, leading to a climactic confrontation that challenges the characters' beliefs and forces them to confront the true nature of their world. The series combines elements of action, drama, and supernatural intrigue, creating a layered and engaging story that keeps readers invested in the fates of its characters.