Ai Amatsu and her twin sister Mai are descendants of a mortal fisherman and a Tennyo, a celestial angel from Buddhist tradition. This sacred lineage designates them as guardians. Trained from childhood by their grandmother, Amatsu Genshu, they study spiritual traditions, martial arts, and the ancient Tenjin dances to prepare for their duties.

Their primary mission is to protect Onimaru, a young half-demon who is a distant relative and the descendant of their angelic ancestor's betrayed younger brother. Since Onimaru’s choices could either plunge the world into debauchery or preserve its current state, his protection is essential. Ai and Mai must combat demonic forces that wield sexual perversion as a weapon and seek to corrupt Onimaru to subvert the human world.

Ai has long brown hair that reaches her hips or beyond and striking purple eyes. She is often seen in her school uniform. As a virgin priestess, maintaining physical and spiritual purity is vital to her role and her effectiveness against the lecherous demons they face.

The conflict pits the twins against demons bearing an ancient vendetta against the Amatsu family for centuries of thwarting their attempts to corrupt humanity. These demons specifically target the sisters, aiming to defeat them through corrupting means. The situation is complicated by the often difficult behavior of Onimaru, who himself exhibits perverted tendencies.

The legacy Ai upholds traces back to a celestial ancestor forced to marry a mortal after he stole her angelic robes, preventing her return to heaven. The twins descend from the two daughters of that union, who founded a temple and an order to control the descendants of their corrupted younger brother. Ai Amatsu continues this family duty, striving to prevent the forces of evil from succeeding.