Sarah Featherstone joined The Grail in response to a tragedy.
Before she joined, she was a Sunday School teacher. She enjoyed teaching children about Jesus, and about God, and wanted to prepare them to live good lives in a hard world. She loved what she did, and she believed strongly in the message she was teaching them.
Then, one day, she came home to find her sister's raped and butchered body in the bathtub. No one was ever caught for the crime.
Most of Featherstone's friends expected her to turn away from God, after having had such an inexplicable and seemingly random tragedy befall her. She was neither stupid or naive, and knew that the world was a terrible place, where terrible things happened. When she saw her sister's body, she just thought God might need something of a helping hand. In her search for purpose, she found The Grail.
The Grail gave her the purpose that she needed - a way to prepare the world for the coming of The Messiah, the true inheritor of the Blood of Jesus Christ. The fact that the Messiah was a horrifically inbred freak was slightly disturbing, however.
So when she was approached by Starr to join his conspiracy, she gladly did so. He wanted to do away with the Grail's debilitated Messiah and replace him with Jesse Custer, a young man who actually did possess divine powers. She believed that Starr knew the right way for the Grail to go, and admired his steadfast devotion to his beliefs, despite immense physical and emotional hurdles.
In fact, she fell in love with him. Her admiration turned to true love for him, and she found herself enduring no end of difficulties for him.
When she confessed her love, however, Starr shut her down hard, ending by saying, "You make a much better adjutant than you ever would a lover, Featherstone." She went back to her work in a daze, but with the rejection of her love she was finally able to see the truth of Starr's nature - he was a monster, both in body and soul.
She tried to kill Starr in San Antonio, but Starr killed her instead.