Cassiopeia Marie Sullivan or "Cassie"– the novel's protagonist, is 16 years old, short, and has curly strawberry-blonde hair. Before the waves hit, she had a crush on Ben Parish, a popular football player. She later has a relationship with her "silencer", Evan Walker.
Evan Walker – a boy from rural Ohio who lived with his large family; he is described as having chocolate brown eyes and brown hair. His family and girlfriend were all killed during the 3rd Wave. He has a brief relationship with Cassie.
Benjamin Thomas Parish/Zombie – a popular football player and honors student at Cassie's high school; he is described as tall, buff, and dark haired. While in high school, Cassie had a crush on Ben, but he never noticed her. After the 3rd Wave, Ben contracts the plague but is "rescued" by soldiers who take him to Camp Haven. There, he becomes a part of Squad 53 and is renamed Zombie. Zombie soon discovers the "soldiers" are actually aliens in disguise. He manages to escape with Cassie and Sammy.
Sissy Parish- Ben's little sister. She was killed by an other while escaping her house with Ben, and he left her behind, he keeps her locket she wore around her neck in her memory. She was described by Cassie to be hoisted onto Ben's shoulders after his football games, and she would join him in a parade. He says he misses Sissy terribly.
Samuel Jackson Sullivan/Nugget/Sammy – Cassie's younger brother who is 5 years old; he is described as having messy brown hair and large, teddy bear eyes. He is taken away from the refugee camp where he, his father, and Cassie had stayed. He is taken to Camp Haven to join Squad 53, after being renamed Nugget. Nugget and Zombie become good friends, and Zombie acts like Sam's big brother. Sammy manages to escape with Cassie and Zombie at the end of the novel.
Ringer/Marika – a girl who joins Squad 53 after Tank, a former member, goes "Dorothy" (slang for crazy); she is described as tall and skinny, with pale white skin and long, glossy, straight black hair, three quarters Asian and a quarter Apache. She is the best shot in the group and is very quiet, yet scary. She and Zombie become friends, and she helps him shoot better. She is often threatening and serious toward the other squad members, but can also be vulnerable at times.
Commander Vosch – the leader of the soldiers and the mastermind behind the plans. He is very sadistic, cruel, and unfeeling. He at first is shown as a "hero" to those at Cassie's refugee camp, but he soon reveals his true nature by personally killing Cassie's father, destroying the camp, and abducting the children, including Sam, to fight. He is also responsible for Camp Haven and becomes familiar with Cassie, Ringer, and Ben Parish.
Dumbo – a member of Squad 53; he is around 12 years old, has huge ears, and is good with medicine and anatomy.
Teacup/Allison – a member of Squad 53; she is seven years old. Originally, she was the only girl, and was the youngest in the squad before Nugget and Ringer arrived.
Poundcake – a member of Squad 53; he is around 8 years old, large and chubby, and rarely speaks, but is good with a gun.
Flintstone – the former leader of Squad 53; he was around 16 years old and had a unibrow. He often challenged Zombie's authority, mainly in jealousy over taking his job as leader. After Zombie reveals Reznik's device, Flintstone takes it and accidentally kills himself with it.
Tank – a former member of Squad 53. He went "Dorothy" (crazy) and was taken away. Dumbo and Zombie discover him dead in the P&D (Processing and Disposal) hangar.
Kenny/Oompa – a member of Squad 53; he was around 8 or 9 years old, chubby, and had crooked teeth. After Zombie sets off an explosion, Oompa is impaled with shrapnel and dies after an unsuccessful attempt to save him.