We only get a glimpse into one day of Genevieve's life, but they are shown to be important details. As Greenbrier, her estate, burned to the ground Genevieve lost her sister, Evangeline, as well as her mother. She was reunited with her long-lost love, Ethan Carter Wate, only to see him get shot and die minutes later. In an effort to revive and resurrect her love with a forbidden spell from The Book of Moons, not only did she fail, she cursed herself and her descendants, meaning that they would not be able to choose whether they turn Dark or Light on their Sixteenth Moon, the choice was made for them.
It can also be assumed that she was once a good, kind-hearted person before she cursed herself with The Book of Moons and became a cruel, cold-hearted woman.
In the movie, Genevieve is portrayed by Rachel Brosnahan.