Rhea

Rhea is th  Titan goddess of fertility and motherhood. She is the mother of Zeus, Poseidon, and Hades, wife of Kronos, and grandmother of Percy Jackson, Thalia Grace, Nico di Angelo and several other demigods, and the great-grandmother of Annabeth Chase and Luke Castellan

The Lightning Thief
She is mentioned when Chiron reveals a fight Zeus and Poseidon had and that they always argued which son their mother favored more.

Physical Appearance
Rhea is said to be a beautiful Titaness with dark hair and green eyes that her son Poseidon and grandson Perseus inherited.