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 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.

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.

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.