Perseus "Percy" Jackson is the Greek demigod son of Poseidon and Sally Jackson, Head Counselor of Poseidon's Cabin and former Praetor of Camp Jupiter. He is Annabeth Chase's boyfriend. Percy has tan skin, messy jet-black hair, sea-green eyes, and a swimmer's build. He is kind-hearted, brave, loyal, and a natural leader, inheriting Poseidon's unpredictable nature. Percy has a sarcastic sense of humor and laid-back demeanor, but can be short-tempered when faced with bullying or manipulation. He dislikes rules and authority, often acting recklessly or speaking before thinking. He tends to blame himself when things go wrong and feels responsible for others. Annabeth considers him obtuse because he can miss obvious hints, especially about people's feelings, though he can be perceptive when necessary. His dyslexia and ADHD make school difficult, contributing to his history of expulsions and strong attachment to Camp Half-Blood. Percy is powerful but gentle, reflecting Poseidon's better nature. Despite his confidence, he carries a quiet sadness and awareness of his dangerous destiny. He hopes the gods can change for the better, and his actions eventually influence them to reconsider their priorities.

Annabeth Chase is one of the Seven, architect of Olympus, former head counselor of Athena's Cabin, and Percy's girlfriend. She has tanned skin, a slender athletic build, long honey-blond hair, and stormy gray eyes. Annabeth is an intellectual genius who is wise, witty, analytical, and passionate about architecture. She is strong-minded, stubborn, determined, daring, and exceptionally brave. Though kind and loyal, she can initially be harsh or judgmental toward people she distrusts. Once someone earns her loyalty, she refuses to abandon them easily, as shown by her continued care for Luke despite his betrayal. Annabeth is extremely level-headed and often hides fear or sadness behind teasing, anger, or determination. She is willing to endure enormous hardship for the people she loves and repeatedly demonstrates courage in dangerous situations.

Grover Underwood is a satyr, Lord of the Wild, Chosen of Pan, and carrier of Pan's soul. He has curly brown hair, brown eyes, horns, a wispy beard, acne, and hairy goat legs. Grover is cheerful, upbeat, clumsy, and big-hearted, though he often underestimates himself. He is extremely loyal to his friends, especially Percy, and pushes through fear when they need him. He can be cowardly and is a terrible liar, but is resilient and dependable. When nervous, he chews on objects, bites his nails, or tugs at his beard or leg fur. He is philosophical and deeply passionate about nature. A vegetarian and environmentalist, Grover hates zoos and is obsessed with coffee, tin cans, and cheese enchiladas.

Niccolò "Nico" di Angelo is an Italian-born Greek demigod, son of Hades, younger brother of Bianca, half-brother of Hazel, and known as the "Ghost King." He is openly gay and Will Solace's boyfriend. Nico has messy black hair, pale skin, dark eyes, and a gloomy appearance. He usually wears black clothing and carries a Stygian iron sword and Bianca's silver skull ring. After Bianca's death, Nico becomes grim, solitary, secretive, and more powerful. He initially blames Percy but eventually forgives him. Nico deeply loves his sister and refuses to abandon those he cares about. Though he feels alienated from his era and other demigods, he remains loyal to the Olympians and repeatedly risks himself to help them. He has tremendous willpower and survives extreme supernatural experiences. He struggles with loneliness, grief, and fear of rejection, especially due to growing up in 1940s Italy. With acceptance from friends like Reyna, Percy, and Annabeth, he becomes more open about himself and his feelings.

William "Will" Solace is a sixteen-year-old Greek demigod, son of Apollo, head counselor of Apollo's Cabin, and a prominent healer at Camp Half-Blood. He is openly bisexual and Nico's boyfriend. Will has shaggy curly blond hair, blue eyes, and an athletic build. He is caring, compassionate, calm, and dedicated to helping others. Although he sees himself primarily as a medic and sometimes underestimates himself, Will is brave and powerful, willingly fighting and even traveling into Tartarus to help Nico. His abilities include healing, archery, light powers, ultrasonic abilities, and plague powers. He is extremely dedicated to his work and tends to overwork himself, sometimes going without sleep for days. Will maintains a calm exterior during stressful situations, though Nico notes that his tendency to overexplain and reassure others usually means he is nervous himself.