Himeno's chest hurts if any boy touches her because her heart is a diamond. However, when she was young, she befriended two boys--gentle Mamoru and blunt Keigo--who could touch her without harming her. Without knowing about her diamond, both Mamoru and Keigo promised to protect Himeno. When some people threaten to take Himeno's heart, who else will protect her but her knights?
After Volume 2 the series is running in Ribon Special! Really! It's not complete!