A daughter of Athena and Percy's closest companion and eventual partner. Brilliant, driven, and deeply competitive, she has been training at Camp Half-Blood since she was seven. Her fatal flaw is hubris - she believes she can do anything. Her arc through the series is about learning that intelligence alone is not enough and that asking for help is not weakness.

A son of Poseidon who discovers his divine heritage at age twelve when monsters begin attacking him. Over five books he grows from a confused kid who can breathe underwater into the demigod at the centre of the Great Prophecy. His fatal flaw is excessive personal loyalty - he would risk the world to save the people he loves. Continues as a major character through Heroes of Olympus and appears in later series.

A son of Hermes and the primary antagonist of the original Percy Jackson series. His resentment of the gods who ignore and exploit their demigod children makes him the vector for Kronos's return. His arc ends with a redemption that the story earns.

A satyr and Percy's best friend, assigned to protect him before either of them understood what Percy was. Deeply anxious and deeply loyal, Grover's great ambition is to find the lost god Pan. His empathy link with Percy becomes a recurring plot device across the series.

A son of Hades who first appears as a cheerful ten-year-old obsessed with a card game and becomes one of the most powerful and isolated characters in the series. His arc across both the PJO and HoO series is about surviving grief, accepting his sexuality, and finding that belonging is possible even for someone who lives between worlds.

A mortal girl with the rare ability to see through the Mist - to perceive monsters, gods, and demigod conflicts that other mortals cannot. Perceptive, artistic, and unimpressed by the world of demigods until she is pulled into it through her chance encounters with Percy.

A daughter of Zeus who was transformed into a pine tree to save her life and restored as a teenager. Her existence violates the oath of the Big Three. Eventually joins the Hunters of Artemis to avoid the Great Prophecy falling to her. Jason Grace's elder sister.

Percy's younger half-brother, a Cyclops son of Poseidon. Enormous, gentle, and enormously strong. Bullied at school for his appearance, he finds his place among the Cyclopes in Poseidon's underwater forges. His devotion to Percy is absolute.