We only get a glimpse into one day of Genevieve's life, but they are shown to be important details. As Greenbrier, her estate, burns to the ground Genevieve loses her sister, Evangeline, as well as her mother. She is 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 is 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 book, she mentioned that she healed a bird, therefore it is believed that she is a Thaumaturge and the last until Ryan. She could also have been a Natural before she turned Dark and became a Cataclyst. In Beautiful Redemption, she was able to Kelt with Lena indicating she is a Cataclyst as only very powerful Casters can kelt.
In the movie, Genevieve is portrayed by Rachel Brosnahan, who is the real-life niece of handbag designer Kate Spade.