Yaezaki Kaoru

Rin Tenkuji receives an old key as a memento on the day of her great-grandmother's funeral. However, that key is a guide to three treasures that will determine the course of the battle (the Aquarian Age) in which girls with superpowers have been vying for supremacy behind the scenes of history!

The protagonist, Rin Tenkuji, is a perfectly ordinary elementary school student who admires his older sister, Sarashina Saiki, a middle school student. One day, Sarashina suddenly decides to move away, and Rin tries to go with her. From there, he becomes entangled in a mysterious fate surrounding the Aquarian Age...