True Mothers - 朝が来る

Kurihara Kiyokazu and Satoko are a married couple. They do not have children. They learn about plenary adoption and adopt a baby son. 6 years later, the couple live happily with their son Asato. They receive a phone call from a woman, who introduces herself as Katakura Hikari. She is Asato's birth mother. She tells them she wants her son back, but, if she can't, she wants money. Kiyokazu and Satoko met Hikari one time in the past. At that time, Hikari was 14 years old. She was a nice girl, who gave them a letter for baby Asato. But when Kiyokazu and Satoko meet Hikari again, there is no image of Hikari as a nice girl anymore. Who is she? What is her purpose?