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 choke out the words due to his grief as he was crying. He 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.
Castor is described as looking just like his father, Dionysus. The only exception is that Castor and his twin brother Pollux both have curly blond hair, instead of Dionysus' curly black hair. They are also described as being plump.
- 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.
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.
- He and his brother are named after the Gemini; Castor and Pollux.
- Like in most versions of the story of the Gemini, Castor dies, but unlike those stories, there are no known attempts to either restore him to life or reunite them.
- Interestingly enough in Greek mythology, the original Castor was killed while the original Pollux lived.
- In the solar system, both Pollux and Castor are stars and both in the constellation Gemini, the twins.
- In ancient Rome, Castor and Pollux had one of the greatest temples, located right in the center of the city.
- In the Latin Culture, Pollux and Castor are known as the Gemini or Castores.
- 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 were named a pair of twins in Greek mythology, in which Pollux was a demigod son of Zeus and Castor was mortal son of Tyndareus, a king of Sparta. They were the brothers of Helen of Troy.
- Pollux and Castor are the names of two brothers in The Hunger Games trilogy as well and incidentally, Castor was killed while Pollux survived.