Ouranos

Ouranos (Ουρανός in Ancient Greek) is the Greek protogenos personifying the sky. Ouranos is the Greek word for sky (a cognate of the English word air), and his Roman counterpart is Uranus. The Greeks imagined the sky as a solid dome of brass whose edges descended to rest on the outermost edges of the earth. Uranus is actually Greek, although the Romans used that name for their primordial lord of the sky. He was the husband and son of Gaea.

History
Ouranos and Gaea were the protogenoi of the sky and the earth respectively. They became consorts and brought forth their children: the Cyclopes, the Hekatonkhires, and the Titans.

Ouranos didn't care for his children, in fact, he hated them. He hated his children that did not look normal most of all, such as the Hecatonkhires and Cyclopes. They were unsightly in his opinion and he wanted them gone. But he couldn't kill them, because they were immortal.

So instead, Ouranos threw the Hekatonkhires and Cyclopes into Tartarus and positioned Kampê to guard them. This caused Gaea great grief. She was also furious because they were her children, and because Ouranos cared nothing for them. She asked her children, the Titans, to get rid of Ouranos so she could save the Cyclopes and the Hecatonkhires. Only her youngest son Kronos was brave enough to do it. He took a flint scythe Gaea made and castrated his father. Nothing was mentioned about Ouranos afterwards. Other accounts stated that Ouranos pushed the Titans back into Gaea because the Oracle said that one of his children would overthrown him.

It is unclear why he never returned or what happened to him because it is impossible for him to die, but since someone is forced to hold up the sky it is very possible that he still exists in some form, some where wishing to embrace Gaea.

The Titan's Curse
Ouranos doesn't make an appearance, but he is mentioned by the General, that a Titan must hold up the Sky because Ouranos is gone, and the Sky still longs to embrace the Earth. Atlas, a Titan, is assigned this torment as of the First Olympian War.

The Battle of the Labyrinth
Ouranos and Gaea are mentioned by Annabeth and a naiad.

The Lost Hero
Coach Hedge mentioned him when he was telling the story of Gaea and how she convinced Kronos to take a scythe and kill Ouranos. When he said that, Piper looked at the sky, as if seeing if it had eyes and a mouth. Maybe wondering of Ouranos, her grandfather.

Personality
Ouranos was a terrible father, and did not care for his children. He was a terrible husband too, because it didn't hurt him that Gaea was so sad about the loss of her children. Though it is possible that Ouranos loves or desires her somewhat, although it is unclear where he is or what he is doing, as stated in the The Titan's Curse by Atlas, who said that "The sky still yearns to embrace the earth."

Abilities

 * Atmokinesis: As Ouranos is the Protagonist of the Sky, he presumably is able to control the weather.
 * He presumably has superior power to the Olympians and Titans (except for the most powerful), seeing as he is an ancient elder deity and was able to throw many of his powerful children (the elder Cyclopes for example) into Tartarus.

Trivia

 * The 7th planet from the sun, Uranus, is named after his Roman form.
 * It is mentioned that Gaea cannot be defeated and only kept asleep, however it is unknown why Ouranos actually died when Kronos cut him to pieces. But because the sky still longs to embrace the earth, Ouranos may still be alive in some form because he is immortal.
 * Ouranos is the father of Kronos, who is the father of Poseidon, making Ouranos Percy's Great-grandfather as well as grandfather to children of Aphrodite.
 * He is the father of the Titans through his wife and mother Gaea, making him the father and half-brother to the Titans.
 * He's the grandfather of the Big Three.
 * The periodic element Uranium is named after his Roman form.
 * Aphrodite was formed out of the sea foam from the pieces of Ouranos thrown into the sea by Kronos.