Sofia Vladimirovna Vassilieva (born October 22, 1992) is an American actress. Her most notable roles include the children's book heroine Eloise in Eloise at the Plaza and Eloise at Christmastime, Ariel DuBois in the Emmy-winning TV series Medium, and teenage cancer patient Kate Fitzgerald in the 2009 film adaptation of My Sister's Keeper by Jodi Picoult.