Orion is the giant son of Gaea born to oppose the twin archers, Apollo and Artemis. Unlike his siblings, Orion doesn't have dragon-like legs and is quite handsome, to the point that he could have passed as an extremely tall human.
Orion was a giant born to oppose Apollo and Artemis. Though not explicitly stated in the series, it is implied that Orion didn't participate in the First Giant War with his brothers, as he was still alive working for King Oinopion of Chios as the royal hunter. However, when Orion attempted to court the king's daughter Merope, Oinopian had him blinded and exiled from Chios.
After settling on the island of Delos, Orion met and befriended Artemis, who was impressed by his hunting skills so much that she allowed him to join the Hunters as the first-ever male. He knew his boundaries around the girls, and kept his distance at all times when necessary.
Soon, Orion's admiration for Artemis grew into love. Fearing that his sister would fall for the giant's charm and turn back on her vows of maidenhood, Apollo drove Orion insane with the urge to hunt every beast on earth. This was out of place with the Hunters' way of life, and it even drew the attention of the sleeping Gaea, Orion's mother. The earth protogenos summoned a giant scorpion to sting Orion, its poison finally putting an end to his life.
After searching for him, Artemis found Orion's lifeless body surrounded by dead critters. To honor her fallen friend, Artemis made a constellation of Orion with the giant scorpion nearby to immortalize his story for all eternity.
He was sent by Gaea to hunt down Nico di Angelo, Reyna, and Gleeson Hedge throughout their journey. He nearly captured them at several points, but they either shadow traveled away before he had the chance, or were saved by allies (e.g., the Hunters of Artemis, the Amazons). He also killed countless numbers of Hunters and Amazons, including Phoebe, who was with Artemis when Orion first lived. However, he was beaten up by the Amazons and Hunters, but escapes with his life. When Reyna is resting with the pegasi that are going to transport the Athena Parthenos to Camp Half-Blood, he shot Blackjack with an arrow, and shot an exploding arrow into the deck of the boat next to them. Reyna prepared to sacrifice herself and kill Orion. She threw her dagger and impaled him through the heart. She jumped on the exploding arrow, setting it off, hoping that her body would take the explosion and that the pegasi will be able to complete the journey. However, as her cloak was infused with the power of Athena, being part of Athena's Aegis, she only feels a slight nudge. Though Orion did not die, Reyna then jumped on Orion's back and strangled him with her blessed cloak, while Bellona, her mother, provided her extra strength. She then threw away his spoil of war, feeling that he deserved to be forgotten.
In The Blood of Olympus, Orion is a ten feet tall (the shortest of the giants), very muscular, and handsome giant, far handsomer than any of his giant brethren, to the point that Orion could have passed as an extremely tall and handsome human, since he is also the only giant without dragon-like or serpentine legs. Orion's skin is the color of wheat toast, while his dark hair is undercut, swept into spikes on top. Due to his bronze mechanical eyes, Orion appears to be wearing military night vision goggles, which are embedded in his eye sockets. As he regards a target, focusing rings on his eyes spin and click, while targeting lasers from them flash from red to green. According to Reyna, Orion can smile so dazzlingly that it can be hard to remember he is a ruthless enemy. He wears black leather breeches, a jerkin, with a hunting knife on his belt, and a black composite bow and quiver on his back, all of which make him look like "Robin Hood's evil, better-looking twin", according to Reyna.
After fighting with the Hunters and Amazons, however, Orion had mottled grey and pink scar tissue on his arms and face, so he looked like a "bruised peach in the process of rotting", while his left mechanical eye was damaged and dark. His hair had burned away, leaving only ragged patches. His nose was swollen and red from the bowstring that Nico had snapped in his face. Much to Reyna's dismay, however, Orion still had his smug smile, with his white teeth flashing in his ruined face.
His huge bow is described as being "almost as fancy as his eyes", with multiple strings running through a series of pulleys that look like miniature steam train wheels. The grip of his bow is polished celestial bronze, studded with dials and buttons.
- Prowess in Battle: Orion is an extremely powerful and skilled warrior, one of Gaea's most dangerous sons.
- Strength: Orion has enhanced physical strength, and easily sent Hylla flying across a street by striking her with the butt of his massive bow.
- Archery: as the bane of both Apollo and Artemis, Orion is an extremely formidable archer, and the the only known one whose skills rival those of the Olympian twin archers.
- Enhanced Vision: Orion's mechanical eyes offer him much better vision than his original eyes ever did, while their targeting lasers make his aim perfect. When he looked at Reyna, she got the uncomfortable impression that he was seeing much more than her form; her heat signature, her heart rate, and her level of fear.
- Water Walking: Orion had the extremely rare ability to walk on the surface of water without sinking, as seen when he approached Reyna in their final confrontation.
- Giant Invincibility: As a giant, Orion can only be killed by a god and demigod working together, and when he is harmed by anybody else, he heals at an accelerated rate. However, as seen in Percy Jackson's Greek Gods, Orion was also vulnerable to the venom of the giant scorpion that Gaea sent after him, ultimately ending in the giant's death. In The Blood of Olympus, Orion is killed again by the combined efforts of Reyna, Bellona, and Athena.
- As the bane of both twin archers, Orion is the only giant to be the bane of two Olympians at once.
- Orion was the first ever male Hunter of Artemis.
- Strangely, Orion's original death was caused by the hands of his own mother, Gaea. Even more odd is the fact that despite being a giant, he was killed by something other than a god and demigod working together (though Gaea may have specifically created the scorpion to be capable of killing Orion).
- In the myths, Orion is actually a son of Poseidon and gifted with the power to walk on water. His parentage is exclusive to the book series. 
- Phoebe mentioned that Zoë Nightshade spent centuries hunting Orion, implying that he reformed sometime after his first death.
- Orion and his half-brother Hyperion are the only two characters who are not related to a sea deity to have the ability of walking on water without sinking.
- The Orion Constellation, one of the most recognizable constellations in the night sky, is named after Orion.