The title character in the coolly engrossing new movie Luce is a high school student who seems exemplary from every angle. Played in a remarkable performance by Kelvin Harrison Jr., Luce Edgar is handsome and popular, an academic and athletic star who gives inspiring speeches at school assemblies. You wouldn't guess that Luce spent his early years as a child soldier in the war-torn country of Eritrea, a trauma that he's clearly worked hard to overcome with the help of his adoptive parents, played by Naomi Watts and Tim Roth, whom he lives with in Arlington, Va.