Number Four is a 15-year-old alien from the planet Lorien who lives on Earth to hide from the hostile Mogadorians. Upon moving to the small town of Paradise, Ohio, he adopts the civilian alias "John Smith." As a member of the Garde, he possesses dormant supernatural powers called Legacies that are just beginning to activate. He struggles to balance his life-or-death responsibilities with his deep desire to stop running and live as a normal teenager.

Henri is a 50-year-old Cêpan (keeper) assigned to protect Number Four and guide the development of his supernatural Legacies. He poses as Four's human father on Earth, using his strong Loric accent to pass as French. Highly disciplined and deeply dedicated to the survival of the Loric race, Henri constantly pushes Four to prioritize his training over socializing and human attachments.

Sarah is a beautiful and friendly student at Paradise High School who recently quit cheerleading to pursue photography. After a summer away in Colorado, she actively distances herself from the mean-spirited popular crowd she used to associate with. Her welcoming nature and genuine kindness immediately draw Four's attention, leading to a quick mutual attraction.

Sam is a gangly, highly intelligent student who wears thick glasses and NASA t-shirts. He is a passionate reader of the extraterrestrial conspiracy magazine *They Walk Among Us* and firmly believes his missing father was abducted by aliens. His nerdy interests make him a frequent target for bullies, but he quickly becomes Four's first loyal human friend.

Mark is the star of the Paradise High School football team and the son of the local sheriff. Highly protective of his social status, he is intensely jealous of anyone who interacts with his ex-girlfriend, Sarah. He uses his popularity and physical size to orchestrate cruel pranks and bully those he views as outcasts or threats.

Bernie Kosar is a scrappy-looking beagle who inexplicably takes a liking to Four and follows him home from school. He demonstrates unusual intelligence and loyalty for a stray dog, constantly keeping watch over Four and Henri while participating in Four's physical training routines.

Number Six is a teenage Loric girl and a fellow member of the Garde. Having lost her own Cêpan years ago, she has survived on her own while actively tracking down the other surviving Loric youths on Earth. She is battle-hardened, fiercely capable, and tired of constantly running from their enemies.

Annie is a friendly local real estate agent in Paradise, Ohio. She helps Henri and Four secure their dilapidated house and immediately tries to integrate them into the community by suggesting Four might become friends with her teenage daughter.

Mr. Harris is the principal of Paradise High School. He handles Four's arrival and occasionally deals with the disciplinary fallout from Mark's bullying, though he often fails to address the root causes of the conflicts in his school.

Kevin is a football player at Paradise High School and one of Mark James's closest friends. He willingly acts as Mark's muscle, carrying out physical attacks and helping orchestrate cruel pranks against outcasts like Four and Sam.

Emily is a student at Paradise High School and a close friend of Sarah Hart. She attends social events with Sarah and inadvertently becomes caught in the crossfire of Mark's pranks against Four and Sam.

Number Three is a member of the Garde who is hunted down and killed by the Mogadorians. His death breaks the protective sequence and triggers a warning scar on Number Four's ankle, signaling that Four is the next target.

Four's grandfather was his primary caretaker on Lorien before the Mogadorian invasion, as Loric tradition dictates that grandparents raise children while parents continue honing their Legacies. He possessed a rare Legacy that allowed him to turn invisible.