Percy is a high school senior and the demigod son of Poseidon. He is currently working to secure three college recommendation letters from Greek gods so he can attend New Rome University in California. He relies on a laid-back, improvisational problem-solving style when dealing with magical crises. He places immense value on his friendships and feels a strong sense of responsibility toward vulnerable creatures.

Annabeth is the demigod daughter of Athena. She is an intelligent architecture student who excels at strategic planning and logical problem-solving. She approaches supernatural challenges methodically and enjoys designing structures. Her intense drive and high standards occasionally cause friction when plans go awry.

Grover is a satyr and a Cloven Elder with a deep connection to the natural world. He possesses the ability to communicate with animals, making him an ideal helper for a magical pet-sitting job. He struggles with hyperactivity when consuming sweet foods and harbors underlying anxieties about his friends moving away to college.

Hecate is the goddess of magic, crossroads, and necromancy. She views modern Halloween celebrations as festivals in her honor and plans a worldwide tour to receive offerings. She enforces a rigid set of rules for her household and frequently places mortals in situations where they must make difficult choices without her guidance.

Peter Stuyvesant is the ghost of a 17th-century Dutch colonial administrator buried in New York. He is summoned to act as an architect but harbors intense prejudices, viewing modern citizens as heretics and seeking to purge the city of things he disapproves of.

Hecuba is a massive hellhound and one of Hecate's prized pets. In ancient times, she was the human queen of Troy who lost her children during the Greek siege. She carries centuries of unprocessed grief and anger, making her highly reactive and difficult to control on walks.

Gale is a magical polecat belonging to Hecate. During ancient times, she was a human witch renowned for her potion-making skills. She retains her vast knowledge of magical brewing and prefers spending her time tearing apart training dummies in her playroom.

Nope is a starving, injured hellhound puppy. He earns his name from the unique sound of his bark. He is highly affectionate and acts as a soothing presence for the older, more aggressive animals in the story.

Mrs. O'Leary is Percy's enormous pet hellhound. She primarily resides in New Rome but travels via shadows to assist Percy when he calls her with a special whistle.

Sally is Percy's mortal mother. Currently pregnant, she provides a steady source of optimistic support for her son. She possesses the rare ability to see through magical illusions and openly shares her experiences dealing with the supernatural world.

Paul is Sally Jackson's husband and Percy's mortal stepfather. He provides a welcoming, ordinary domestic presence that grounds the demigods after their supernatural crises.

Eudora is a Nereid who functions as Percy's high school guidance counselor. She is highly anxious and easily intimidated, especially regarding anything involving Hecate due to her complicated history with the goddess.

Chiron is an immortal centaur who acts as a mentor for demigods. He operates under a mortal disguise at Percy's school, offering crucial historical context regarding Greek spirits and artifacts.

Filomena is a naiad and former handmaiden to the magic goddess Circe. She operates a successful perfume business in Manhattan and holds a grudge against Percy for past actions involving her former home.

Silbe is a naiad who utilizes magical perfumes as weapons. She harbors a grudge against demigods and attempts to paralyze Percy using an ancient spray.

Daedra is a naiad twin who runs a magical scent shop. She exploits captive magical creatures for profit and relies on aggressive transformation potions, including one that turns her into a bear demon, during physical conflicts.

Phaedra is a naiad twin and a former handmaiden to Circe. She utilizes dangerous, unpredictable potions in battle, viewing her magical attacks as justified revenge against the demigods.

Juniper is a dryad and Grover's girlfriend. She offers a calm, grounding presence and reassures Grover about the enduring nature of their friendships despite upcoming college moves.

Clovis is a demigod who joins a chaotic supernatural battle in New York. He utilizes a gentle, empathetic approach to resolve conflicts with angry spirits rather than relying strictly on force.