The younger sister of Chiharu and her opposite in more than one way.
Yuriko is Chiharu's younger sister.
She is almost the exact opposite to her sister for two reasons. One is that unlike her sister, mother, and for some reason her brother (you cannot say what was going through the editors head when they thought that up without calling them a freak), her chest is not very developed with only an A cup through the series to compare to her sister's original F cups which grow through the series.
The other difference is that unlike her sister she not only openly confesses that she likes Densuke but she constantly makes amorous advances towards him. Often jumping him in such a way that it deliberately press's her crotch against his face as well in the anime where she moves his hands to her breasts and holding his wrist in place so he can't pull away.