–Halcyon, before dying in The Demigod Diaries
Halcyon, also known as "Hal," was born with ability to see the future. Apollo warned him to never talk about his predictions as it would anger the gods. But he met a young girl who owned a dagger who was destined to die in an accident, but Hal saved her by telling her the future. In return for his help, she gave him the dagger to thank him. By telling her the future, the gods were angry and Apollo cursed him. Hal was forced to wear the skin of the Python, who guarded the Oracle as a reminder that he wasn't an oracle. He was stripped of his voice and locked inside his childhood home, a mansion in Richmond, Virginia, where he was guarded by the leucrotae who were linked to his thoughts and could speak for him. They kept him alive as bait for future demigods, who would look for a treasure, as a way for Apollo to remind him that his voice will lead others to their doom. After the demigods were eaten, he would keep their food and other supplies they had traveled with.
He decided to create a diary, filling it with his notes and predictions of the future.
One day, Thalia and Luke were led by Amthelia to Hal's mansion. Upon finding out that Hal trapped them as bait for the Leucrotae, they tried to escape before the sun sets, which they eventually did, by making Greek fire, a dangerous substance. Hal however sacrifices himself to the leucrotae. He gave his diary to Luke.
The Diary of Luke Castellan
Now an old man, Luke Castellan and Thalia Grace stumble across Hal's mansion and the leucrotae. He saves them temporarily by calling them up into his room, but explains that soon the leucrotae will get in and devour them, and that the entire mansion is a trap. Hal feels remorse for them and knows that no demigod can escape his cursed mansion. Despondent and pessimistic of their chances at first, when he discovers that they are the ones to gain the treasure many demigods have lost their lives trying to obtain, he tries to be useful by helping them search for a way to defeat the leucrotae. Once Thalia receives the treasure, a bracelet that turns into Aegis, they discover the previously lost recipe for Greek Fire, and discover that they can create some from ingredients in the room, planning to use it to destroy the leucrotae.
While trying to foresee Thalia and Luke's future, Hal sees that Thalia will sacrifice herself for her friends, and one day find her family again, (possibly Jason). He also saw that Luke would betray his friends one day, causing Thalia to be angry with him, saying that Luke would never betray anyone. Hal gives Luke his diary, a dagger he received from the girl he had saved, and makes him promise that he will stand up to gods, something he wish he had done, and to learn from his diary so that Luke will not make the mistakes that he, Hal, made. Hal decides to sacrifice himself and lures the leucrotae into his bathroom, before setting off the Greek Fire explosion and dying, finally finding peace.
Halcyon is described as a tall and gaunt old man, about sixty, with spiky gray hair, slumping shoulders, liver-spotted hands, and sad green eyes underscored with bags. He wears greenish-brown snakeskin boots and clothes made from the mighty Python that guards the Oracle. According to Luke, Halcyon might have looked handsome once, but the skin of his face now hangs loose, as if he is "partially deflated." His facial expression is so full of misery, that he looks like he's close to tears. Overall, he looks like a "very old, sickly, and fashionably dressed Einstein." Halcyon's voice is heavy with sadness, and feels alone and miserable, like a hostage.
- ADHD: Like all demigods, Halcyon possesses inborn supernatural battle reflexes and senses that he uses to analyze the fighting style of his opponent.
- Dyslexia: His brain is hard-wired for reading Ancient Greek instead of modern languages.
- Foresight: Being a child of Apollo, the god of oracles, Halcyon has the rare ability to see into the future, though he and Idmon are the only known sons of Apollo to have inherited this ability.
- Archery: Being a child of Apollo, Halcyon is a naturally talented archer, his skills only rivaled by his aunt, Artemis' maiden hunters.
- Vitakinesis: Being a child of Apollo, Halcyon can heal people by singing to his father in Ancient Greek
- Couplets Curse: He can curse others to only speak in rhyming couplets that can take days or even weeks to wear off.
- Enhanced Athleticism: Being a child of Apollo, Halcyon is skilled in physical contests and games.
- Musical Aptitude: Being a child of Apollo, Halcyon is a natural musician.
Halcyon made three known predictions:
- Young Girl's Prediction: Halcyon predicted a young girl would die in an accident during her youth.
- Thalia's Prediction: Halcyon predicted Thalia would survive the attack of the leucrotae and that she'd sacrificed herself to save Annabeth, Grover, and Luke, and that she would be turned into a tree and sleep for years alone. Then, she would be reawakened and return to her physical form. After being reawakened, she would gain immortality due to joining the Hunters of Artemis. Her path was sad and lonely after being returned to the present and she would later meet Jason, her brother she thought she had lost again.
- Luke's Prediction: Halcyon predicted Luke would survive the attack of the leucrotae as well, but couldn't see his path clearly. He only knew that Luke would eventually betray his friends and the gods, but would ultimately sacrifice himself to make sure the gods paid attention to their children, that he'd choose to become Kronos, and that he would betray his friends at Camp Half-Blood. Hal's predictions wasn't clear, however, which is why Luke was so anxious to hear what Hermes knew about his fate when he finally meet his father shortly thereafter.
- Halcyon is a Greek name, most of the time considered a girl's name. It means "kingfisher" and "calm", after a fabled bird, identified with the kingfisher, that was supposed to have had the power to calm the wind and the waves while it nested on the sea during the winter solstice. It also means "quiet", which alludes to Hal Green's muteness.
- Green is a common surname of English origin.
- Halcyon was the oldest known living son of Apollo.
- Halcyon in the only known son of Apollo to have the extremely rare gift of prophecy, and can see into the future when he concentrates enough.
- In the The Diary of Luke Castellan, it is revealed that Halcyon Green had made a 14-year-old Luke promise that he would learn from his mistakes, something Luke misunderstood to mean not to be pushed around by the Olympian gods. Thus, Halcyon was the first one to inspire Luke to question the gods, which led him to ally himself with Kronos and the other Titans in the Percy Jackson and the Olympians series.
- His name is also a shade of green.