Percy is a 17-year-old demigod and the son of Poseidon. He possesses water-manipulation abilities and struggles academically with dyslexia and ADHD. He hopes to attend New Rome University with his girlfriend, but he learns he must first complete dangerous quests to earn three divine recommendation letters from the gods.

Annabeth is an intelligent daughter of Athena who attends the School of Design, New York City. She shares Percy's academic learning differences but utilizes her encyclopedic knowledge of Greek mythology to formulate strategic plans. She intends to attend New Rome University and commits to helping Percy complete his challenging quests.

Grover is a satyr who serves as Percy's long-time best friend and protector. Though he acts like a teenager, satyrs age slowly, placing him in his thirties. He lives at Camp Half-Blood and uses magical pan pipes to manipulate nature or distract monsters during dangerous encounters.

Ganymede is a physically beautiful minor god who serves as the divine cupbearer on Mount Olympus. Originally a mortal, he received immortality from Zeus but actively dislikes his subservient position. He lives in constant fear of Zeus's temper and desperately needs a stolen magical chalice returned before the next divine feast.

Hebe is the goddess of youth and a former cupbearer to the gods. She manages a massive arcade called Hebe Jeebies, which utilizes nostalgia to make its patrons physically younger. She takes the form of a teenage girl and actively enforces a rule requiring her to be the youngest person in any room.

Iris is the goddess of the rainbow and a former divine messenger. She dresses in a grandmotherly hippie style and sells crystals and incense. She facilitates communication for demigods through Iris-messages and requires a specific favor involving her herald's staff before providing information to Percy.

Poseidon is the Greek god of the ocean and Percy's father. He remains largely absent from Percy's daily life and is forbidden from offering direct help due to ancient pacts regarding powerful demigod children. He occupies his time renovating his underwater palace while offering his son minimal guidance.

Sally is Percy's mortal mother who lives with him in Manhattan. She provides a stable, loving environment that contrasts with the life-threatening chaos of Percy's demigod duties.

Paul is Percy's human stepfather. He understands Percy's supernatural reality and offers practical, grounded advice about the natural human experiences of aging and family.

Eudora is a Nereid who functions as a guidance counselor at Alternative High. She informs Percy of the strict requirements for his college application and utilizes water vortices to instantly transport him to various locations.

Geras is the god of old age, taking the physical form of a highly decrepit elderly man. He believes all humans should age naturally and aggressively opposes the concept of immortality for mortals.

Elisson is a minor river god who resembles a yoga instructor. He fiercely defends his territory and attempts to drown intruders who enter his polluted waters without permission.

Blanche is the mortal daughter of Iris. She dresses entirely in black and works as an aloof photographer who strictly shoots dead subjects in black and white, deliberately contrasting her mother's rainbow aesthetic.

Sparky is a 65-year-old mortal employee who physically resembles a nine-year-old child due to the reverse-aging magic of the arcade. She strongly advocates for the benefits of eternal youth.

Zeus is the king of the gods and the god of lightning. He imposed the unique recommendation letter requirement on Percy and demands absolute perfection from his divine servants during his elaborate feasts.

Athena is the goddess of wisdom. She generally disapproves of Percy but occasionally offers subtle assistance when her daughter's interests align with his quests.

Juniper is a dryad involved in a romantic relationship with Grover. She occasionally experiences jealousy regarding his outside activities and modeling sessions.

Naomi is a cloud nymph who assists demigods in secretly navigating the restricted areas of Mount Olympus during divine banquets.

Barbara is a dryad who agrees to help smuggle items through Mount Olympus in exchange for an introduction to Annabeth.