Saturn

Saturn is Kronos' Roman counterpart. Saturn's wife was Ops (the Roman equivalent of Rhea). He was the father of Ceres, Jupiter, Vesta, Pluto, Neptune, and Juno, among others. In the Roman tradition, in memory of the Golden Age of man, a mythical age when Saturn was said to have ruled, a great feast called Saturnalia was held during the winter months around the time of the winter solstice. It was originally only one day long, taking place on December 17, but later lasted one week.

The Son of Neptune
It is revealed by Jason that while the Greek demigods were fighting Kronos in the Battle of Manhattan, the Roman demigods were fighting Titan Krios and monsters on Mount Othrys.

The Son of Neptune
Saturn's comeback attempt is mentioned by Nico. The first day Percy - having had his memory taken by Juno - arrives to Camp Jupiter, Nico almost calls Saturn by his Greek name (Kronos) while telling Percy the story of the Battle of Mount Othrys. Nico is shown to be very cautious when telling the story as if fearing the return of Percy's memory.

Trivia

 * Saturn had a temple on the Forum Romanum which contained the Royal Treasury.
 * Saturn is the namesake of both Saturn, the planet, and Saturday (dies Saturni).