Keiko Sakai is Sana's birth mother. She is quite young and looks a lot like Sana, and she has quite a nervous character.
Twelve years after she abandoned Sana on a park bench, She called the Kurata Mansion after 'My Daughter and I' was published. She went and met with Sana and told her everything that had happened. They met once more in the anime, and Misako met with Keiko once briefly after that. But after that Keiko had no more appearances.
In the Manga Edit
Keiko's uncle impregnated her but she kept it a secret from her family because she loved her uncle so much. She didn't want to hurt her family if they found out that she became pregnant and ended up giving birth to Sana in a bathtub when she was only 14 years old.
In the Anime Edit
When Keiko was growing up, her mother was never around much: she was always busy with her job or boyfriend. Keiko never knew her father, and it was hard for her to be alone all of the time. She began dating an older an named Takeshi Goji, and although she was young, she really loved him. and the feelings were mutual. When she found out she was pregnant, she couldn't tell anyone, even him. She was frightened and didn't know what to do, so she had the baby all alone. Takeshi disappeared without a word before this happened, and she presumed he was dead.
Later, in her twenties, she met and married a kind and understanding man, who didn't mind at all of Keiko's past when she told him. They had a little girl named Mariko.
Mariko is Keiko's youngest daughter. She is a huge Sana fan, and is unaware that Sana is her older sister. Keiko has stated that
Sana is Keiko's first child, but abandons her right after she was born, leaving her crying in a park bench. Misako later hears a child crying and adopts her. After discovering Sana again, Keiko asked Sana to live with her once, but Sana said she already had a mother. She thanked Keiko for bringing her into this world and that said that she and Keiko wouldn't be seeing each other again.