Fukamine Riko♀

Riko is Haruhiko's younger sister in name, although they are the same age and are in the same class together in school after she enrolls. In third grade, she joined Haruhiko's family due to family circumstances and Haruhiko immediately treated her as if she really were his little sister. When they were younger, she left for several years, only to appear again one day out of the blue years later. The reason she returned was to be with Haruhiko as he is her first love and has continued to be in love with him. She is currently a second-year at Shimano Academy and shares the same class as Haruhiko.

[From Wikipedia]