Mother Curie III

Mother Curie III used to be a member of the Church of the Children of Atom, the religious group in Megaton that worships the atomic bomb sitting in the middle of the town. In the Broken Steel DLC, Project Purity begins chugging out clean water for the Capital Wasteland inhabitants. When shipments of Aqua Pura arrive in Megaton, Mother Curie III, along with members of her newfound cult that lives just outside of Megaton, hijack the water and irradiate it. They then begin handing out the irradiated water, which they call "holy water". 
 
Mother Curie III is a pacifist. Even if the player murders Brother Gerard, who hangs around outside the entrance to the church (it's located in the basement of a destroyed house), she won't be hostile and will simply flee if she herself is attacked.