Hornet Sonia was the daughter of Ludwig with his first wife. How Ludwig was not a god at the time of his birth, Sonia is not a demigoddess. When her half-brother, Eden, has borned, Sonia has sworn to protect him, and so followed the Mars and Medea plan, which was to become the new king Eden on Earth.
So, Sonia became a Mars Army member (wearing the Hornet Galaxy), and in the first mission, killed Kasuma, who was the father of the future Lionet Saint (Souma). Sonia is just fulfilling its mission, very hurt inside, hating the act did; but it was all for Eden. Gradually, Sonia was getting colder and merciless of killing. She is as manlike as Shaina.
Souma tried to avenge his father, but Sonia manages to escape alive. His brother betray Mars, Medea tells Sonia wear Scorpio Cloth and try to convince Eden to return to ally the father. But, Souma (that had already forgiven her) get there earlier, and asks Sonia to live their own life, without having to take responsibility for anyone. Sonia mocks the Souma face, and uses her most powerful technique, however, this ends up leaving her for dead, clueless space where it is; Souma tightens Sonia hand, and regrets his death, which occurs shortly after.
Mars and Eden lament Sonia's death, but Medea simply feels nothing for her death, and she was treated as a battle object, as she did with Mars and Amor.