Rhea

Rhea was known as 'the mother of gods', queen of the Titans, daughter to Gaia and Uranus. Rhea was wife to Kronos and mother to Demeter, Hades, Hera, Hestia, Poseidon, and Zeus.

History
Kronos, Rhea's Titan brother and husband, castrated their father, Uranus. After this, Kronos re-imprisoned the Hekatonkheires, the Gigantes and the Cyclopes and set the monster Kampê to guard them. He and Rhea took the throne as King and Queen of the Titans. This time was called the Golden Age.

Kronos sired several children by Rhea: Hestia, Demeter, Hera, Hades, Poseidon, but swallowed them all as soon as they were born, since he had learned from Gaia that he was destined to be overcome by his own child as he had overthrown his own father.

When Zeus was about to be born, however, Rhea sought Gaia to devise a plan to save him, so that Kronos would get his retribution for his acts against Uranus and his own children. Rhea gave birth to Zeus in Crete, handing Kronos a stone wrapped in swaddling clothes, which he promptly swallowed.

Then she hid Zeus in a cave on Mount Ida in Crete. He was raised there by his grandmother Gaia, until he was old enough to take on his father, with the Titaness Metis on his side.

Zeus forced the Titan Kronos to disgorge the other children in reverse order of swallowing: first the stone, which was set down at Pytho under the glens of Parnassus to be a sign to mortal men, then the rest. In some versions, Metis gave Kronos an emetic to force him to disgorge the babies, or Zeus cut Kronos' stomach open. Then Zeus released the brothers of Kronos, the Gigantes, the Hecatonkheires and the Cyclopes, who gave him thunder and lightning, which had previously been hidden by Gaia. Zeus and his siblings, together with the Gigantes, Hecatonkheires, and Cyclopes, overthrew Kronos and the other Titans. Similarly, in later myths, Zeus would swallow Metis to prevent the birth of her child, Athena, but she was born unharmed, out of a wound made in his head by one of the other gods.

Most often Rhea's symbol is a pair of lions, the ones that pulled her celestial chariot and were seen often, rampant, one on either side of the gateways through the walls to many cities in the ancient world. The one at Mycenae is most characteristic, with the lions placed on either side of a pillar that symbolizes the goddess.

The series
It is unknown in the books what happened to Rhea after the war but it can be assumed like any immortal she is still alive. She seemed to have managed to keep out of her husband's Second War.

It would appear she keeps in touch with her children, as in The Lightning Thief. Chiron mentions her when talking about Zeus and Poseidon quarrelling, one of there usual arguments envolves into which one of them she liked best.