This woman was the mother of Mitsuko Souma. It is assumed that she has sex with men for money but eventually she met a pedophile man who wanted her young daughter. Seeking to earn more money, she agreed to pimp Mitsuko out after she was out of school.
When the young Mitsuko arrived home, she thought her mother was sick and tried to get away from the perverted man. Mitsuko's mother told her daughter to be strong and not like her. Moments later she heard a loud noise and to her shock, she seen that Mitsuko had pushed the man downstairs, killing him.
What happened afterwards between the mother and daughter is unknown, but it did changed the way Mitsuko viewed things and she was never the same. Mitsuko's mother's fate is unknown. She may have passed away from poor health sometime after Mitsuko killed the pedophile.