A very shy and insecure young woman.

She gave birth to Sana when she was only 14. In the manga it is revealed that her uncle made her pregnant, and she gave birth in a bathtub. Keiko says that she was very much in love with Sana's birth father, but that he has died. When Misako finally meets her, after Keiko offers proof about her bond to Sana, she slaps Keiko and says that even animals know they have to take care of their offspring. She also arranges Sana to meet her. Sana was worried about meeting Keiko, because she was afraid that Misako would give her up; she finds out that Misako had the same worry, that Sana would leave with Keiko. After meeting Keiko, Sana and Misako conclude that it's what in the heart that defines your family, and that they are truly mother and daughter. After meeting Sana, Keiko asks if it is possible that one day Sana would come to live with her. Sana gently but firmly says that she considers Misako as her mom, although she's grateful to Keiko for bringing her into the world.
(Source: Wikipedia)