Koios

"These Giants may think they will rule. Let them be our shock troops and destroy the Olympians - all well and good. But once the Earth Mother is awake, she will remember that we are her eldest children. Mark my words. The Titans will yet rule the cosmos."

- Koios to Iapetus

Koios (also known as Coeus) is a son of Ouranos and Gaea. He is the Titan of farsight, knowledge, and is the Titan of the North. His Roman form is Polus.

History
Led by his brother Kronos, Koios worked with his brothers Iapetus, Hyperion, and Krios to restrain their father Ouranos, while Kronos slew him with his scythe. After the Titans took control of the world, Koios ruled as the Lord of the North and the Titan of farsight. He married his sister Phoebe, with whom he had his daughters Leto, the mother of Apollo and Artemis, and Asteria, mother of Hecate.

In the First Titan War, the Olympians, the immortal children of Kronos and Rhea, fought against the Titans. To Koios's disappointment, his peaceful daughter Leto played no part in the war. Kronos was slain by his son Zeus with his own scythe. In the ensuing war, Koios was defeated and banished to Tartarus with the rest of his brethren.

The Last Olympian
After Kronos turned Luke Castellan against the Olympians, the Titans were slowly resurrected to take on the gods. Koios was one of the Titans who aided Kronos in the Second Titan War. Kronos and Hyperion led the Battle of Manhattan. Oceanus led the assault on Atlantis. Krios defended Mount Othrys. Iapetus attempted to overthrow Hades. It is unknown, however, what Koios' part in the war was. After Kronos was dissolved by Luke, Koios, along with the other Titans, were sent back to Tartarus.

The House of Hades
After the Doors of Death were captured and opened by his mother Gaea, Koios was among the Titans and monsters who planned to return to the mortal world. Koios was reunited with his brother Iapetus, now known as Bob. During the Titan War, Iapetus's memory had been destroyed by Percy Jackson, when he tackled him into the River Lethe. In the Second Giant War, Iapetus defected to the side of the Olympians. Even as Tartarus healed his memory, Iapetus continued aiding Percy Jackson and his girlfriend Annabeth Chase, who had been thrown into Tartarus in a fight in Rome.

Hidden by the Death Mist, Percy and Annabeth were unseen by Koios. The Titan assumed Iapetus's memory was returning and that he was returning to their side. Iapetus didn't counter him. Koios reminded Iapetus about their uprising against Ouranos, remembering how their father kicked Iapetus in the face. Koios planned to return in order to complete the Titans' takeover of Mount Olympus, believing that Gaea would remember that the Titans were her eldest children and forget about the Gigantes. He also hoped to persuade his daughter Leto to join them in their conquest, believing that she will agree to side with the Titans, after the way Zeus treated her after the birth of Apollo and Artemis. He hurried off, assuring Iapetus that he would meet up with their brothers Hyperion and Krios at the entrance to the Doors of Death.

Koios's plans were thwarted when Iapetus betrayed the Titans and aided Percy and Annabeth against the Titans, monsters, and the lord of the pit, Tartarus. With the help of the peaceful Gigante Damasen, Iapetus held the elevator's button, knowing that Percy and Annabeth were needed in the war against Gaea. Due to Iapetus's and Damasen's sacrifice, Percy and Annabeth returned from Tartarus and the Doors of Death were closed, imprisoning Koios inside Tartarus.

Appearance
Koios was roughly twenty feet tall with elaborate black Stygian iron armor and a single diamond blazing in the breastplate. His eyes were blue-white and as cold as a glacier. His hair was blue-white, cut in military style. He wore a helmet shaped like a bear and wielded a massive sword the size of a surfboard, which radiated "more cold than Hubbard Glacier". His face, though marked by a number of battle scars, is quite handsome, and according to Percy, Koios had the same cold eyes as Artemis, with the same smile as Apollo, as he is their grandfather.

Personality
Not much is known about Koios. He is quite cruel and arrogant, like the rest of his Titan brethren, since he believed that his race, the Titans, were superior children of Gaea. The Titan felt that the Titans were better than the Giants and the Olympians alike. However, he seems to be on good terms with his fellow Titan brothers, as he seems upset about Kronos permanently losing his body, and greeted Iapetus enthusiastically, cheerfully recounted their conquest against their father, and vows revenge on Hades for enslaving him. He also seems to love his daughter Leto, lamenting that she would not join the Titans' cause, but has no such affection for his grandchildren Apollo and Artemis or his nephew Hades. He is also quite eloquent in his manner of speech, making it sound like he was "reciting Shakespeare" to Percy.

Abilities
No abilities of Koios have been shown, but the following can be inferred:
 * Prowess in Battle: As an Elder Titan, Koios is very powerful, though not as much as his brothers Hyperion and Kronos. He, apparently, actively participated in the second Titanomachy, though he was finally defeated. His weapon of choice implied that he was an excellent swordsman.
 * Titanic Energy: Like all Titans, Koios has the ability to blast things hundreds of yards away from himself with a yell or a wave of the hand.
 * Titanic Divine Form: As Kronos's elder brother, it is possible that Koios also had the ability to incinerate any being less than a god/Titan only by being present.
 * Cryokinesis (possibly): As the Titan of the North, Koios might have the ability to manipulate snow, ice and frost, as his sword was said to have radiated "more cold than Hubbard Glacier."  

Trivia

 * Koios was not well-known in Greek mythology, and even less in Roman mythology, where he was known as Polus. He was mainly mentioned on a list of Titans. He only considered prominent in the history of the Greco-Roman deities.
 * Koios is the only known Titan that never appeared and was never even mentioned in the first book series.