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 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 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 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 Pluto - the Roman aspect of Hades - who died in 1942 and was resurrected by Nico. She can sense and control precious metals and gems underground. Her arc deals with guilt over her role in releasing Alcyoneus in her first life and her relationship with Frank Zhang.

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.