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 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 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.

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.