Mai Amatsu is the younger twin sister of Ai Amatsu. Both are virgin priestesses descended from a celestial Buddhist angel and a fisherman, charged with protecting Onimaru. Onimaru descends from a lineage capable of becoming a great warrior of light or a demon king. This duty originates from their ancestors, the original twin sisters who founded a temple order to restrain the descendants of their betrayed younger brother. That brother's vengeful spirit now seeks to transform Onimaru into a depraved demon king to unleash debauchery.

Mai has shoulder-length brown hair, distinguishing her from her sister, purple eyes, and the appearance of a teenage girl. Her personality is more fickle than Ai's, and she displays a stronger attraction to good-looking individuals.

To protect Onimaru and combat the demonic minions serving the founding brother's spirit – minions who spread sexual perversion and seek to corrupt Onimaru – Mai wields a naginata. She fights alongside her sister Ai, their grandmother Genshu Amatsu, and Onimaru's guardian Dekunobo. Their mission is to prevent Onimaru from fulfilling this dark destiny while countering the demonic threats.