KastorCastor

He and his brother, Polydeukes (Polydeuces or Pollux), were called the Dioskuri (Dioscuri); they were the twin sons of Leda and Zeus and the brothers of Helen, Klytemnestra (Clytemnestra) and Phoibe (Phoebe).

In The Iliad (book 3, line 243) Kastor and Polydeukes were merely mortals but later stories gave the brothers a more supernatural countenance; as examples:

1. When Helen was a young girl, she was kidnapped by Theseus and Pirithous; Kastor and Polydeukes saved her with the help of Akademus (Academus) or perhaps Dekelus (Decelus);
2. While they were with the Argonauts, the two brothers became involved with the daughters of Leukippus (Leucippus), Hilaeira and Phoibe (Phoebe), and, for one reason or another, Kastor was killed; Polydeukes was supposedly immortal and did not want to live if his brother was dead; Zeus had mercy on the devoted brothers and allowed Kastor to return from the land of the dead on the condition that Polydeukes would take his place; that meant that, alternately, one of the brothers would be condemned to the Underworld while the other would be free on the face of the earth; eventually they were introduced into the heavens as the constellation, Gemini, i.e. the Twins.

An elaboration of the death of Kastor survives in the fragmentary remains of the Kypria; the author (not Homer) says that the two brothers were caught stealing the cattle of Idas and Lynkeus (Lynceus); Kastor was killed by Idas and then he and his brother, Lynkeus, were killed by Polydeukes; Zeus made Kastor and Polydeukes immortal with the condition that while one of them lived on the surface of the earth, the other would reside in the Underworld.