Percy Jackson and the Olympians: Legends

Koios was roughly 10 feet tall, with an 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 and beard were blue-white, with his hair cut in military style. He wore a helmet shaped like a bear's head, 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 Jackson, Koios had the same cold eyes as Artemis, with the same bright smile as Apollo, as he is their grandfather.

Koios is revealed to be extremely intelligent, wise and capable of predicting the future. As a result, Koios was frequently approached by all of his siblings, asking him questions. Apparently, even Kronos held Koios in great esteem, and he didn't question Rhea's decision to give birth to Zeus on Crete when she claimed that the clairvoyant Koios advised her to. Koios is also quite cruel and arrogant, like the rest of his Titan brethren, since he believed that his race, the Titans, were superior children of Gaea and more sophisticated than the Giants and the Olympians alike. However, Koios seems to be on good terms with his fellow Titan brothers, as he seems very upset about Kronos permanently losing his body, and greeted Iapetus enthusiastically, cheerfully recounted their conquest against their father. He also seems to love his daughter Leto, 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

Koios was the second son born to Gaea and Ouranos, the respective personifications of the earth and sky (after Oceanus). After Kronos claimed his supremacy as the Titan King of Mount Othrys, and ushered in the "Golden Age", Koios was justly rewarded for his help with control over the North. Koios was one of Kronos' most loyal followers, held in great esteem by Kronos himself. Koios was also the only Elder Titan who was naturally clairvoyant, and could accurately predict the future. As a result, all of Koios' siblings, nephews and nieces would frequently approach him with questions about the future. 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. Years later, 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 through the doors.