Clarke is a teenage resident of the privileged Phoenix section of the space colony who was training to be a doctor like her parents. She is imprisoned for allegedly aiding her parents with illegal radiation experiments, a trauma that shatters her trust in the colony's government. When sent to Earth, she assumes the role of head medic for the 100 teens, putting the lives of her patients above her own.

Wells is the privileged son of the colony's chancellor and a former guard-in-training. He intentionally gets himself arrested by setting the colony's only surviving tree on fire so he can accompany Clarke to Earth. Once on the planet, he attempts to use his training to establish order and authority, but the other teens reject him because of his association with his father's strict rule.

Bellamy is a resident of the impoverished Walden ship who forces his way onto the transport to protect his younger sister. Having spent his entire childhood taking rash risks to hide her existence from the authorities, he acts as her primary caretaker. On Earth, he quickly adapts to the freedom, becoming a skilled hunter and a reliable provider for the group.

Glass is a Phoenix resident who receives a rare pardon and remains on the space colony while the other teens go to Earth. She was originally arrested for breaking the strict population control laws regarding an unplanned pregnancy. Her experiences as a prisoner and her relationship with a boy from the lower-class Walden ship fundamentally change her perspective on the colony's class divisions.

Octavia is Bellamy's younger sister and an unregistered child whose existence violates the colony's strict one-child policy. She grew up hidden in a closet, enduring severe abuse from her mother and constant terror of discovery. On Earth, she struggles to adjust to her newfound freedom, occasionally acting selfishly because she was denied basic needs for so long.

Luke is a resident of the lower-class Walden ship and Glass's boyfriend. He has endured a difficult life on the impoverished section of the colony, giving him a pragmatic and grounded outlook. Despite the harsh inequalities he faces, he maintains his maturity and devotion to Glass, even when their relationship faces strict social and physical barriers.

The Vice Chancellor is a corrupt and manipulative leader on the space colony. He actively abuses his power, forcing citizens into illegal acts and executing those who stand in his way. When the Chancellor is incapacitated, he seizes control of the colony, using his new position to protect his own privilege while sacrificing the lower-class ships.

Graham is one of the 100 delinquents sent to Earth, having been convicted of murder on the colony. He sees the lack of adult supervision as an opportunity to seize power, using fear and intimidation to establish himself as a leader. He frequently clashes with Wells and Bellamy over the distribution of resources and the rules of their new camp.

The Chancellor is the leader of the space colony's ruling council and Wells's father. He enforces the Gaia Doctrine with absolute strictness, believing that harsh measures are necessary to keep the surviving population alive. He is shot and incapacitated during the chaotic launch of the 100, leaving a power vacuum on the colony.

Glass's mother is a wealthy resident of the Phoenix ship. She is deeply concerned with social status and appearances, immediately focusing on integrating Glass back into high society after her pardon. She strongly disapproves of her daughter's relationship with a boy from the lower-class Walden ship.