"The girl" is Katniss Everdeen and Peeta Mellark's daughter. She has a younger brother, who is referred to as "the boy". Their first names are unknown as they were never mentioned by the author in the epilogue. She has long dark hair like Katniss, and blue eyes like Peeta. According to Katniss, the girl has learned about the Hunger Games in school and has an idea that her parents played an important role in them.
In Catching Fire, Peeta lied about Katniss being pregnant to try to protect her from the Games. Katniss said that she miscarried due to an electric shock in the Arena. However, 15 years after the events of Mockingjay, Katniss became pregnant for real. Katniss describes terrible fear at the feeling of her daughter moving inside her during her pregnancy. The only medicine to heal this fear was finally holding her baby in her arms. At first, Katniss did not want children at all, but Peeta pined for them and she gave in. It is shown in the epilogue that Katniss truly loves her children, and loves that they can live without the Games. However, she is remorseful, knowing that someday she will need to tell her children about their parents' role in the Games and the rebellion. The girl already knows that they had a role in the Games, but one day, Katniss knows she will have to tell her more.