The First Sister by Linden A. Lewis

First Sister has no name and no voice. As a priestess of the Sisterhood, she travels the stars alongside the soldiers of Earth and Mars—the same ones who own the rights to her body and soul. When her former captain abandons her, First Sister’s hopes for freedom are dashed when she is forced to stay on her ship with no friends, no power, and a new captain—Saito Ren—whom she knows nothing about. She is commanded to spy on Captain Ren by the Sisterhood, but soon discovers that working for the war effort is so much harder to do when you’re falling in love.