The culprit is Toshiko Betsusho. She explains that she is not Kazuki's mother. However she did knew her and was aware that she was sending Kazuki monthly postcards. Betsusho got jealous, because she couldn't have children of her own, and thought the mother's attempt to connect with her abandoned son was pathetic, so she blackmailed Kazuki's production company, in the hopes that they would stop pass on the postcards to Kazuki. The story of his father being a murderer was a lie, and she did learn the details of Kazuki's birthmark and the name of the church, where he was left, from the real mother, which made it possible for her to make the scam work. However she never imagine that they would actually pay her the money. And when Kamoshita turned up and threaten to blackmail her, she killed him.
