Castor

Castor was a Greek demigod son of Dionysus and the twin brother of Pollux. He deceased during the The Battle of the Labyrinth.

The Lightning Thief
Castor is mentioned by Percy. Castor 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.

The Battle of the Labyrinth
Castor fights bravely for the camp and dies in The Battle of the Labyrinth by the hands of an enemy half-blood ( he was stabbed in the arm and hit on the head). His father Dionysus is depressed and angry over his son's death, surprising everyone by caring about his dead 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.

The Last Olympian
In Percy's vision, Dionysus states that he had already lost one son (Castor) and didn't want to lose another one, and asks Percy to keep Pollux safe.

Relationship with Pollux
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 are 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.

Abilities
Castor could control the states of the mind, and control grapes and strawberry plants, just like his father. This ability is called Chlorokinesis.

Trivia

 * 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.