Who is Isabelle Derby from Copper Sun and what is their importance?

Isabelle Derby is Mr. Percival Derby's second wife. She is eighteen-years-old, and she is described as a kind woman, who treats the slaves with respect. Her treatment of the slaves is undoubtably a result of her love affair with Noah, which produced a child. When Isabelle's baby is born, she immediately knows that Mr. Derby will kill her, so she begs the slave girls to hide both her and the baby. Her plan, however, fails, and Mr. Derby shoots both Noah and the infant..... killing them both.