Castor is first mentioned by Percy Jackson. He is seen first with his brother helping the strawberry plants grow, then at dinner he is seen at table 12. Percy notices that Dionysus only has two boys in his cabin, Castor and his twin brother Pollux. However, their names are not yet mentioned.
Castor fights bravely for the camp, but dies during the Battle of the Labyrinth by the hands of an enemy demigod (stabbed in the arm and hit on the head). His father Dionysus is depressed and angry over his son's death, surprising everyone at camp by caring about his dearly departed son.
At Castor's funeral, his twin brother Pollux tries to make a speech for his brother, but is unable to recite the words, choking due to his grief. Pollux then lights the funeral pyre and sobs.
In Percy's vision, Dionysus states that he had already lost one son, Castor, and did not want to lose another one, and asks Percy to keep Pollux, his twin brother, safe for him.
Percy had mentioned Castor in The Lightning Thief. He described them as 'a couple of plump blond boys who looked just like Mr. D', suggesting that they look a lot like Mr. D and may not be as athletic as the other campers. However, it is stated that they are good athletes. The noticeable difference between Mr. D and the twins is the color of their hair. Mr. D has black hair while the twins have blond hair.
- ADHD: Like most demigods, he possesses inborn supernatural battle reflexes and senses that he uses to analyze the fighting style of his opponent.
- Dyslexia: His brain is hardwired for Ancient Greek.
- Chlorokinesis: As a son of Dionysus, he can control plants, grape and strawberry vines.
- He can presumably control madness, being able to cure or cause it. However, it may not be so as he could not care for Chris Rodriguez, who was inflicted with madness. Much later, Dionysus cured Chris to overcome the pain he got when Castor had died. It could also be that he wasn't powerful enough to cure Chris's madness.
It is shown that they were very close, and that they loved each other. Pollux was brokenhearted at his brother's death, and this helped his fury against Kronos and his monsters, so he had no doubts about him fighting for his brother's memory. They were always seen working together in the camp. Percy is also sad that he never really got to know Castor before he died as stated at the end of The Battle of Labyrinth.
Dionysus wasn't shown to care for his son Castor while he was alive. However, Dionsyus was shown to be really sad when Castor died. In fact, Castor's death made Dionysus more protective towards Pollux, Castor's twin. In the Tower of Nero it is revealed that Dionysus actually Lived with Castor, Pollux and their mother for a while and enjoyed it. From that experience Dionysus told his brother Apollo that two is the perfect number because anymore and the kids start to outnumber the adults. It is unclear how long Dionysus stayed with the family or if Castor and Pollux had interacted with their father much while they were at Camp Half-Blood together. If they didn't interact much at camp that leads one to wonder if now that Dionysus didn't have their mother with them he felt outnumbered by the two boys.
- Pollux and his brother were named after a pair of twins in Greek mythology, known as the Dioskouroi — Pollux (whose original Greek name was Polydeukes) was said to be a son of Zeus, born to him by the mortal queen Leda, while Castor was a regular mortal, the son of Leda's husband, King Tyndareus of Sparta. They were also the twin brothers of Clytemnestra, wife of King Agamemnon, and Helen of Troy.
- Like in most versions of the story of the Dioskouroi, Castor dies, but unlike those stories, there are no known attempts to either restore him to life or reunite them.
- Interestingly enough, in the original myth, the original Castor was killed while the original Pollux lived. Afterwards, the two brothers had their images placed in the sky as a constellation, which was named Gemeni ("twins") in honor of them. (Gemeni is the name by which they were known in Latin).
- In Ancient Rome, the original Castor and Pollux had one of the greatest temples, located right in the center of the city.
- In The Battle of the Labyrinth, Percy admits he is ashamed that he saw Castor around camp for three years, but never bothered to learn his name.
- Pollux and Castor are the names of two brothers in The Hunger Games trilogy as well and incidentally, Castor was killed while Pollux survived.