Gaia

Gaia (Γαία in Ancient Greek) (sometimes spelled Gaea and prounced Gi-a) is the primordial Greek goddess personifying the Earth and the leading antagonist of the second series (The Heroes of Olympus). She was one of the first beings to arise from Chaos at the beginning of time. Her Roman counterpart is Terra. Terra is the Latin/Roman word for earth. She's very cruel, considering that every living lifeform is living on her even though she's willing to kill her husband, son(s) and grandchildren.

History
Gaia/Gaea and her husband and son Ouranos were two of the first Greek (gods, giants and titans) to emerge at the beginning of the world. She was the goddess of the Earth, and he was the god of the Heavens. They became partners, and brought forth the Titans, their children.
 * Oceanus
 * Koios
 * Krios
 * Hyperion
 * Iapetus
 * Theia
 * Rhea
 * Themis
 * Mnemosyne
 * Phoebe
 * Tethys
 * Cronos

And The Hundred Handed-Ones
 * Kottos
 * Briares
 * Gyes

Gaia/Gaea also gave birth to the first Cyclops as well as the Hekatonkheires. Along with them, she was also the mother of the Protogenos Pontus, the primordial personification of the ocean. She also fathered many of his children, as well as children of the Olympians and other Protogenoi. She is also the mother of the monster Typhon, whom she bore through a coupling with Tartarus.

After Ouranos pushed the Hekatonkheires in Gaia's womb and the Cyclops into Tartarus, Gaia was furious. She asked her children to get rid of Uranus so she could save her other children, and only Kronos was brave enough to do it. He took a scythe Gaia made, and got rid of his father.

However, Kronos didn't rescue his brothers, and Gaia was then angry with him. Gaia/Gaea then prophesied that Kronos in turn was destined to be overthrown by his own son, and so the Titan attempted to avoid this fate by devouring his young. Zeus, through deception by his mother Rhea, avoided this fate, and later rescues the Hekatonkheires from Gaia's womb and the Cyclops from Tartarus, and forced his father to vomit up his brothers and sisters.

A war then started between the Titans and the gods, Hekatonkheires, and Cyclops. The young gods emerged victorious, and banished Kronos into a place called Tartarus with bonds that they will never break.

Gaia's first attempt to avenge the titans was, with the spirit of Tartarus, giving birth to Typhon. Typhon, being the father of all monsters, was one of the worst enemies Zeus and the Olympians had to face. Typhon, with the help of Echidna, was able to render Zeus helpless and imprison him in a cave. However, with the help of the other gods, Zeus, with his lightning bolts, was able to trap Typhon under Mt. Etna. Gaia/Gaea then made peace with the Olympians and promised not to bother the Olympians again.

However, that was not Gaia's last attempt to overthrow the Olympians. She prophesied that Zeus's wife Metis would bear two children, first a daughter, and second a son who would eventually overthrow him. To avoid this fate, Zeus convinced Metis to transform into a fly, and then swallowed her. But it was too late. Metis already conceived a child. A few days later, Zeus received severe headaches. He convinced Hepheastus to axe his head open and out came a matured Athena. It is unknown if Metis escaped or not, but she never conceived the second child, so Gaia's plan failed. However, prophecies cannot be avoided, so it is likely that we have not heard the end from the son destined to overthrow Zeus.

It was revealed that Gaia/Gaea mated with Poseidon which resulted in the birth of the half-giant Antaeus. Zeus even mated with Gaia which resulted in the birth of Orion and King Manes: Father of Atys.

Some time later, Gaia/Gaea used the Giants she has been harboring during the First Olympian War to avenge the Titans by attacking Mount Olympus only for them to be defeated by the Olympian Gods and Demigods.

The Battle of the Labyrinth
Gaia/Gaea is mentioned by Annabeth in the Battle of the Labyrinth when Percy battles her son, Antaeus (Also a son of Poseidon), in an arena deep within the Labyrinth. At first Percy is unable to harm him, because any wound he receives is instantly covered by sand and healed by Gaia's power. Percy is able to defeat him by causing him to be disconnected with the Earth, rendering his mother unable to protect him.

A river nymph also mentions her when she tells Percy that the area around her river, including Geryon's stables, were once part of the ocean when only Gaia/Gaea and Uranus reigned. This references the first age of the world, before the Titans ruled.

The Lost Hero
Gaia/Gaea appears spirtually when Leo Valdez was little and leaving his mom's work and caused the place to burn down. Leo was alive becuase of his ability to resist fire, but his mom didn't make it out. So Gaea might have been very weak but begining to wake, when Leo was little. Gaia/Gaea also appear during Jason's Quest to save Hera,She appeared again to Leo Valdez It is officially revealed that Gaia is awakening and using the Giants to try and overthrow the current rulers of Olympus, the Olympians as she did after the first First Olympian War.

Personality
Gaia is the mother of the Titans, and the grandmother of the gods (most of the gods but not all). She loves her children very much, and was furious with Uranus for throwing them in to Tartarus, even willing to give up having more to save them, only for Kronos to conduct a similar act with his children. After the Titanomachy, she also sent her greatest son, Typhon, to war with the Olympians. As well, she also instigated the Gigantomachy, in which she persuaded the Giantes to rise up against the Olympians. So, in retrospect, she is responsible for the usurpation of her husband and for two attempts on the Olympians.

Powers
Gaia is the eldest of all the Primordial gods, goddesses, Titans, and Titanesses. Being the all-encompassing mother of Earth, she can manipulate the Earth and sense everything that occurs on or within it. She does not appear physically in the series, so her exact influence and powers are mysterious. Mythology presents Gaia as seldom taking a physical form, her spirit instead remaining within the Earth itself. In The Lost Hero it's said that if she were to rise in her full physical form she would be more terrible and powerful than any other force on the living planet.