Keiko is the mother of the Shinonome children. Except to Miharu to whom she is an aunt.
She has the largest bust in the family and the series at 123cm, but said that they were about the same size as Chiharu's at her age. In the same picture which showed this also revealed she was Chiharus age in 1985. Since the manga was realesed in 2001 that would put her at least at 31, meaning she had Chiharu at a young age.
Keiko is a single mother; later in the manga she admits to Densuke that she has feelings for him despite being significantly older than him. She intially attempts to seduce Densuke after but stops after realizing that her daughter is truly in love with him.
She is more like Miharu in the sense that she is not embarressed by her body but is nowhere near as blatant as Yuriko, since she does not purposely thrust her body onto Densuke but on the occasions that he sees her in a compromising position or accidentally gropes her, Keiko does not mind and even encourges him without throwing herself at him.