Who is Lunetta Brogan from Blood of the Fold and what is their importance?

Lunetta Brogan is Tobias' sister. She has the gift of magic. Although Tobias has taught her that it is the taint of the Keeper, Lunetta uses her magic to serve both her brother and the keeper. When Tobias goes insane, Lunetta is forced to kill him. She is then killed by his man, Galtero.