Alexis is a young woman who spent her childhood in an abusive foster home before a blood test reveals her Spartan heritage. Thrown abruptly into the deadly initiation massacre and the crucible, she struggles to survive in a brutal society of immortals. Despite her traumatic past, which left her with heavy scarring and a deafened, discolored eye, she remains highly protective of those she loves.

Kharon is the heir to the House of Artemis and the ferryman of the River Styx. Known as "The Hunter," he is a formidable Chthonic Spartan who was forced to fight in the Spartan Gladiator Competition and serves in the Assembly of Death. He is scarred by an unexplained trauma, possessing a permanent leg injury and a "furia" tattoo, and commands invisible hellhounds as his animal protectors.

Augustus is the heir to the House of Ares and a strict professor in the crucible. Famed for his honorable public persona, he holds rigid, archaic views on gender and honor, making him furious about a woman participating in the deadly Spartan trials. Beneath his fair exterior, he harbors a vicious brutality that he unleashes privately to enact his own version of justice.

Patroclus is a Spartan "mutt" of the House of Aphrodite and half of the famed Crimson Duo. He possesses the Chthonic power to detect lies through physical touch. Publicly perceived as an unstoppable killing machine alongside his partner, he uses a confrontational demeanor as a defense mechanism against social harassment regarding his sexuality.

Achilles is a Spartan "mutt" of the House of Ares and the other half of the Crimson Duo. Because he possesses a death-dealing voice, he perpetually wears a muzzle to protect those around him. Like his partner, he is targeted by societal prejudice, prompting him to maintain a standoffish public persona while striving for the elite rank of general.

Theros is an Olympian and the current heir to the House of Zeus. As an ambitious member of the Spartan elite, he is highly protective of his status and presents an intimidating presence among the younger initiates. He possesses the power to erect an invincible, defensive shield, though it has specific proximity limitations.

Zeus is the leader of the Spartan Federation and the most powerful of the remaining Olympians. Arrogant and self-important, he holds immense sway over Spartan society and the deadly trials the youth must undergo. He is highly invested in preserving his faction's dominance and expanding his House's prestige.

Hades is the most powerful and respected Spartan in the Chthonic faction. Despite leading the charge against the Olympians during the Great War, he now coexists uneasily within the Spartan Federation. He projects an aura of danger and commands dark elemental forces during the trials.

Helen is the sister of Patroclus and Augustus. As one of the rare female Spartans her age, she is heavily sheltered by her overprotective Chthonic brothers but dreams of proving herself. She finds inspiration in Alexis's endurance and reaches out to offer support.

Charlie is a nonverbal human boy who shares an abusive foster home with Alexis. The two forge an unbreakable bond, surviving starvation and trauma together and eventually relying entirely on each other while living in a cardboard shelter in the forest.

Nyx is an invisible echidna, a snake-like creature whom only Alexis can see and communicate with. He serves as her best friend and constant companion during her horrific upbringing in the human world and attempts to comfort her during the grueling Spartan trials.

Drex Chen is a 19-year-old Spartan initiate who survives the initiation massacre. Having also experienced abandonment in his childhood, he forms an alliance with Alexis, offering physical protection in the trials in exchange for her tutoring him in math.

Maximum is an enthusiastic 19-year-old Spartan initiate participating in the crucible. He is openly friendly towards Alexis in an environment where most are actively hostile, bringing unwanted attention from the instructors.

Titus is a cruel 19-year-old initiate who repeatedly targets and bullies Alexis during the crucible. He often instigates physical fights and uses intimidation tactics against weaker students in his cohort.

Christos is a Spartan initiate from the House of Zeus who attempts to form a connection with Alexis early in the crucible, mistakenly believing they share the same Olympian lineage and can help each other survive.