Himeno has a diamond for a heart and every time a person of the opposite sex touches her directly her chest becomes unbearable with pain. But when she meets Mamoru and Keigo she has no pain at all! She wonders why this happens but then finds out they are her knights, protectors of the diamond inside her heart, because it turns out whoever can posses her mind and body is allowed to have a wish granted by the diamond. The pain in her chest vanishes as her body becomes mature. She attends the same school as Keigo thinking Mamoru attends this school but turns out Mamoru attends a different school in France just with the same name.
She eventually grows feelings for Keigo, and confesses at the end of the manga.