ゆりね ～おねえさまがおしえてくれた～

One day, the protagonist's older sister died. Upon hearing this, his mother went mad and began referring to the protagonist by his sister's name, "Natsuki." The protagonist slowly accepted this role to make his mother happy. His mother even went as far as to enroll him in the school she used to go to, Momoyuri Academy-- an all-girls school.

Momoyuri Academy is a famously prestigious school, where the daughters of company directors, politicians, bureaucrats and the like attend. Within the student body lies the descendent of the Kamikura zaibatsu, Kamikura Sayaka. A graceful young lady excelling in schoolwork and sports, Sayaka is the object of every student's admiration, including those of the so-called "Natsuki."