Keto

Keto (also spelled Ceto or Cetus) is one of the most powerful ocean monsters in Greek Mythology. She was mentioned by Chiron in The Titan's Curse.

General
Keto is the daughter of Pontus and Gaea. She is most notable for bearing by her husband as well as her brother Phorkys, an old sea god, The Gorgon sisters Medusa, Euryale and Stheno, at the same time Phorcydes, the Graeae (Graiai), also known as the Gray Sisters, Echidna and Ladon.

History
The queen of Äthiopia, Cassiopeia, once said she would be more beautiful than the Nereids. Angry because of this, Poseidon sent Keto to cause trouble in Äthiopia. To calm Keto it was decided to sacrifice Cassiopeia's daughter Andromeda. She was chained to rocks and had to see as Keto came closer. Before the monster struck, Perseus appeared at the cliff and fell in love with Andromeda with one look. Andromeda told him that she had begged her fiance to rescue her but, cowardly, he left. Perseus killed the monster and brought Andromeda to safety. Promptly he asked Andromeda's parents for her hands. Her parents promised him not only Andromeda, but the whole kingdom, for they were grateful for the life of their daughter.

There are a few different versions of the way Perseus killed Keto. In one version he showed Keto the head of Medusa, so that Keto turned into stone. In another version he had to battle Keto and killed her with a magical sword from Hermes.

Keto in Ancient Texts
It is said that Keto had children with Phorcys, as Echidna, the Gorgons (Medusa, Euryale, and Stheno and sometimes Scylla), and Ladon are their offspring. It is also said Typhon is Keto's grandson.

Trivia

 * It is said to be one of the most powerful monsters by Chiron.
 * In The Lightning Thief (film), when Percy was asked by Chiron if he knew any other demigods, he looked at a mural with the name Perseus defeats Cetus, and thus knew the answer.
 * The asteroid 65489 Ceto was named after her and it's satellite after her husband.
 * Like Typhon being the most powerful Wind Monster, Keto is considered to be the most powerful Ocean Monsters.
 * Ironically, Keto is one of the only monsters Percy never set eyes on. Seeing as Percy is a son of Poseidon, a fight between Percy and Keto would be well matched.