<P> At the base, the military have used deception and technology to convince the rescued children that the humans outside the base have been possessed . They provide military training to the children, forming them into squads to go on kill missions outside the base . Sam has been placed into a squad led by Ben, a boy Cassie had a crush on, along with Ringer, a tough teenage girl, Dumbo and Teacup . While out on a kill mission, Ringer removes her military implant, causing her to register as an Other - possessed human on the squad's scopes . The squad deduces the plan to have them kill real unpossessed humans, making them the 5th Wave . Ben sends his squad into the woods and returns to base, claiming his squad was killed, for the purpose of retrieving Sam, who was left behind . </P> <P> Ben confronts Colonel Vosch about child warriors being the 5th Wave and Cassie kills Sergeant Reznik during her one - on - one indoctrination spiel . Ben and Cassie find each other and leave to find Sam . Evan sets off numerous bombs and advises them to quickly find Sam before the destruction of the entire facility . Colonel Vosch and the surviving military evacuate with the human children by military aircraft . Cassie, Ben and Sam escape, with help from Ringer, just as Evan completes the destruction of the base . Ben's squad is reunited and Cassie ponders the strength of hope as humanity's driving force for survival . </P> <Ul> <Li> Chloë Grace Moretz as Cassie Sullivan </Li> <Li> Nick Robinson as Ben Thomas Parish </Li> <Li> Ron Livingston as Oliver Sullivan </Li> <Li> Maggie Siff as Lisa Sullivan </Li> <Li> Alex Roe as Evan Walker </Li> <Li> Maria Bello as Sergeant Reznik </Li> <Li> Maika Monroe as Ringer </Li> <Li> Zackary Arthur as Sam Sullivan </Li> <Li> Liev Schreiber as Colonel Alexander Vosch </Li> <Li> Tony Revolori as "Dumbo" </Li> <Li> Talitha Bateman as "Teacup" </Li> <Li> Nadji Jeter as "Poundcake" </Li> <Li> Alex MacNicoll as "Flintstone" </Li> </Ul> <Li> Chloë Grace Moretz as Cassie Sullivan </Li>

Who plays the girl in the fifth wave