Ai is one of the two title characters from the adult anime series Injuu Seisen: Twin Angels. She is a teenager who, along with her younger sister Mai, is a student at Tenjin Girls High School. Ai is the older of the two Amatsu sisters. She has brown hair that reaches past her hips, which she typically wears in a long ponytail, and purple eyes.

Ai and Mai are descendants of an angel and a fisherman. In the traditional Japanese folklore that forms the series' basis, the fisherman stole the angel's robes while she bathed, forcing her to marry him. The couple had twin daughters, who are the ancestors of Ai and Mai. Those original twins founded a temple and an order dedicated to restraining the destructive impulses of their younger brother, who had been betrayed by the Japanese government and whose spirit seeks revenge.

As a result of this lineage, Ai is a virgin priestess whose role is to watch over a boy named Onimaru, a distant relative. Onimaru is the descendant of the vengeanc-seeking younger brother, and his choice will determine whether the world becomes a place of debauchery or remains as it is. Along with her sister, Ai fights against the demonic forces that serve Onimaru's corrupted ancestor. The sisters are assisted in this struggle by their grandmother, Amatsu Genjuu, and by Dekonobu, Onimaru's guardian.