Jen was Kristie's closest colleague in the army. Bree disliked her because she ripped off her arm and nearly succeeded in burning it once.
When Riley called Kristie forward as a group leader, she was reluctant to step up. Riley then called for Jen instead and stated that she would become one of the newborn leaders since Kristie would not step up. Hearing him sat that, Kristie pushed Jen out of the way and stood proudly next to Riley. Jen was hinted to be in Kristie's "group" and had a very hard time going out in the sun, since she and the rest of the army were told the stereotypical stories about vampires.
At the end of the book, she was destroyed along with the rest of her army.