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 Mars - the Roman aspect of Ares - with the ability to transform into any animal. Descendant of Poseidon through the Chinese demigod tradition. His life is literally tied to a piece of firewood. One of the most genuinely kind characters in the Riordanverse.

A daughter of Pluto - the Roman aspect of Hades - and one of three Roman heroes given the prophecy that opens The Son of Neptune. Quiet, watchful, and carrying a past she rarely talks about, Hazel can sense and control precious metals and gems underground - a gift she finds as much a burden as an advantage. She fights alongside Frank Zhang in the Fifth Cohort at Camp Jupiter.

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 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, particularly when it comes to his mother and the workshop fire that took her.

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 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.

Goddess of the hunt and the moon. Leader of the Hunters of Artemis, an immortal band of young women who swear off romantic love in exchange for eternal life. One of the few Olympians who treats demigods with consistent respect.

Goddess of wisdom, craft, and battle strategy. Annabeth Chase's mother. Born fully formed from Zeus's head. Her relationship with Poseidon is a persistent hostility that complicates Percy and Annabeth's relationship.

A Titan's daughter cursed to remain on Ogygia and fall in love with every hero who washes up on her island. Released from the curse by Leo Valdez, she spends the Trials of Apollo series adjusting to mortality.

A satyr who works as a protector of demigods, disguised as a physical education teacher at the Wilderness School. Aggressive and combat-loving, he wields a baseball bat-turned-club and calls everyone 'cupcakes'. Despite his tough exterior, he is deeply loyal and protective.

A massive bronze dragon automaton built by the Hephaestus cabin at Camp Half-Blood. Repaired and given wings by Leo Valdez, Festus serves as a loyal mount and later as the figurehead of the Argo II warship, his consciousness preserved in the ship's systems.

The primordial Earth Mother, older than the Olympian gods. She is slowly awakening from millennia of slumber, using her giant children and mortal agents to destroy the gods. She speaks through the earth itself, manipulating events across the world.

God of the Underworld and king of the dead. Nico and Bianca di Angelo's father. Perpetually aggrieved at his exclusion from Olympus politics, but not actually evil - one of the series' recurring jokes is that Hades is treated as a villain when he usually isn't.

Queen of the Amazons and elder sister of Reyna. A formidable warrior who grew up on Circe's island and later rose to lead the Amazon nation. She secretly helps the demigods despite political pressures within her organisation.

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.

The augur of Camp Jupiter and a legacy of Apollo. A thin, pale young man who reads prophecies by gutting stuffed animals. Ambitious and manipulative, he seeks the praetorship and opposes cooperation with the Greek demigods, ultimately leading a rogue attack on Camp Half-Blood.

The king of the giants, born to oppose Zeus/Jupiter. An enormous figure with gold and silver skin who was raised from the earth at the Wolf House. He is the leader of Gaea's giant army and the primary antagonist in the final battle at the Parthenon in Athens.

God of the sea, earthquakes, and horses. Percy Jackson's father. One of the Big Three who swore not to father children after World War II. His relationship with Percy is defined by distance and periodic, costly interventions.

Praetor of Camp Jupiter and one of the most capable leaders in the series. Daughter of Bellona, Roman goddess of war. Her arc in Blood of Olympus is one of the series' best subplots.