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 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 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 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.

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 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 daughter of Ares and Percy's rival at Camp Half-Blood. Aggressive, proud, and often antagonistic, she is also genuinely brave when it matters. Her role in the Sea of Monsters is a turning point for her character.

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.

A mortal girl with the ability to see through the Mist who becomes the host of the Oracle of Delphi. Perceptive, artistic, and unimpressed by the world of demigods until she is pulled into it. Becomes the Oracle permanently at the end of The Last Olympian.

A son of Hephaestus with the rare ability to generate and control fire. The comic relief of the Heroes of Olympus ensemble who turns out to be carrying more grief than he lets anyone see. His subplot in Blood of Olympus and Dark Prophecy is one of the most satisfying payoffs in the series.

A daughter of Aphrodite with the power of charmspeak - the ability to make people do what she says. Self-conscious about her heritage in a series where beauty and love magic are considered less impressive than combat powers. Her arc is about redefining what strength looks like.

A son of Jupiter - the Roman aspect of Zeus - and praetor of Camp Jupiter. Wakes at the start of The Lost Hero with his memory erased. His arc through Heroes of Olympus is about the tension between Greek and Roman identity and what it means to lead. Thalia Grace's younger brother.

A daughter of Demeter with control over plants and seeds, assigned as Apollo's master when he is cast down to earth. Difficult, unpredictable, and deeply traumatised by her upbringing under Nero. Her relationship with Apollo is the emotional core of the Trials of Apollo series.