Well, Reiko is Kakyuusei's ojousama.

Reiko has the attitude of some ojousama. She thinks she is superior to everyone. However, Tsuyoshi does not let Reiko treat him like that, but rather as similars. Her episode is episode 8.

She only makes a small cameo in the OVA, saying a line.

Note: "ojousama" is usually referred to the 'debutante' high society girls.