Ryuunosuke Fujita would like nothing more than a fun, carefree life with his friends. But life doesn't always go how you want it. When the third-grader arrives home one day, his father introduces him to Koharu Mori, a young wheelchair-bound girl with prosthetic limbs who from now on will be Ryuunosuke's younger sister. For Ryuunosuke, this means that he must care for her at every moment and will have no time for himself. Though sympathetic to Koharu's situation, Ryuunosuke comes to resent her and the responsibility that threatens to rob him of his youth.

However, Ryuunosuke soon finds his mundane reality replaced by fantasy. When he finds Koharu floating in the air one night, a nightmarish creature from his imagination suddenly attacks them. After his father dispatches the monster, he reveals that Koharu is actually a divine being; her very presence can materialize the monsters that come from the innate fears of others—a power so great that it could lead to the end of the world if used in the wrong hands. With the help of Toukaishuu, an alliance dedicated to dispatching monsters, Ryuunosuke vows to protect Koharu and fight against those who intend to use her abilities for evil.

Volume 3: Momo Monogatari: Toshiharu to Tomodachi-hen