Prometheus

Prometheus (Προμηθεύς in Ancient Greek) is the Titan of crafty counsel and forethought. He is known for giving humans the gift of fire. He is the son of Iapetus and Clymene.

History
Prometheus sided with the gods in the first war because he knew that the Titans would lose. As a result, he was not imprisoned. However, because of his meddling nature, he stole fire for humans, which boosted their progress as a species. This made Zeus angry, so as punishment for giving humans fire, Zeus had Prometheus chained up on a mountain and had an eagle peck out his liver every day (though Prometheus' liver regrew every night). This would be evaded if either someone breaks the chains or if Prometheus tells Zeus which of his fellow gods would overthrow him. Prometheus was later freed by Heracles who also slew the eagle.

The Sea of Monsters
Prometheus is first mentioned in the Percy Jackson series by Hermes, when he meets Percy in the middle of the night. He was interrupted from his chat with Percy by a phone call, describing Prometheus's state, as he had been chained to a rock with vultures pecking at his liver, only to have it grow back again. Hermes had said to the person on the other line, "Yes, I know that he is chained to a rock with vultures pecking at his liver, but we can't possibly locate his package without a tracking number."

The Last Olympian
In The second Olympian war, Prometheus joined Kronos in the battle against the gods, and was sent as a messenger by the Titans in an effort to persuade the demigod forces into surrendering. Claiming to have always been an ally of humanity, he tells Percy Jackson that he has foreseen their defeat, and asks them to give up their fight. He claims that the gods are just as petty, if not worse, than the Titans and that humanity would benefit from the victory of Kronos. To tempt Percy, he claims he has a new revelation that would carry humanity as far from its current point as fire brought them, but that under Zeus mankind would never be allowed to advanced that far. Percy declines, but Prometheus leaves behind Pandora's Pithos or Pandora's Jar, saying that should he change his mind Percy would need release Elpis, the spirit of hope who resided within the jar, to symbolically "give up Hope" and show their surrender.

In battle, he fought for Kronos. Otherwise, he was neutral and works with the winning side. He was a part of the truce party when they confronted Luke. When the Titans lost, Prometheus claimed he was merely pretending to be on Kronos' side so as to minimize bloodshed. This possibly shows that Percy and Luke Castellan were able to defy his thinking, or alternatively, the indignity of being tied up and having his regrowing liver eaten by a vulture every day impaired his judgement of the gods. It is even possible he did what he did as part of a grander scheme, possibly involving the future. More likely is that he rather calculated, then perceived the future (as he is the titan of Forethought but not necessarily foresight) so he probably joined the Titans because they had a higher chance of winning. Nevertheless, he goes into hiding and has sent a list of excuses for helping Kronos to the Big Three. Hermes states that if Prometheus knows what's good for him, he won't show his face for a few centuries.

Appearance
He wears a tuxedo (hence why Percy called him the "tux dude" when he meets him on a diplomatic meeting in The Last Olympian), being over 2 meters tall, wears a ponytail and has strange marks on his face that looked like some small animal scratched it (Most likely talon marks from the Eagle or a really, really mad hamster as Percy thinks).

Trivia

 * A moon of Saturn is named after Prometheus.
 * The element Promethium is named after Prometheus.
 * A minor plot hole in The Sea of Monsters is that when Hermes mentioned someone "chained to a rock with his liver being torn out" and "a gift for mankind", referencing Prometheus, despite the fact that he was rescued by Heracles centuries ago.
 * Prometheus has an apparent phobia of vultures. When Percy mentioned them, Prometheus flinched and said to Percy that he would rather Percy not to mention the vultures. This may have occurred because of Prometheus' Curse.
 * Percy refers to him as "Tux-Dude" in The Last Olympian.
 * Even though he is the Titan of forethought, he was unable to see Kronos lose, so it is possible he just joined Kronos for revenge