Jessica Sanders is the name of Niki Sanders' adopted sister, who was killed in 1987 by their adoptive father Hal. Later Niki's doppleganger takes on the persona of Jessica when using her super strength. When Jessica is revealed, Niki becomes somewhat trapped in her mirror. By the end of the first chapter, Niki seems to have her control over Jessica and seeks help to get rid of her second personality. It should be known that Jessica is just a personality created from the reaction of Niki seeing her sister abused, and not at all the true Jessica Sanders possessing her body.
Jessica Gravestone

Niki and D. L. visit the grave of Niki's sister, Jessica.