Castor and Pollux ("beaver" and "sweet wine") were the twin sons of Leda and Zeus/Tyndareus and the brothers of Helen of Troy and Clytemnestra in Greek mythology. They joined Jason and the Argonauts in the quest for the Golden Fleece. When Helena was abducted by Theseus, they invaded Theseus' kingdom of Attica to rescue their sister.
Later, a feud with their cousins, Lynceus and Idas, led them to leave Helen behind alone to entertain the guests so they can steal their cousins' herd. This exposed Helen to the seduction (or abduction) by Paris of Troy, which eventually started the Trojan War.
The cattle theft ended in a bloody conflict, leaving Lynceus and Idas dead, but not before Idas dealt a mortal wound to Castor. Since the twins had different fathers, this meant that Pollux (sired by Zeus) was immortal and Castor (sired by Tyndareus) was mortal. When Castor died, Pollux asked Zeus to let him share his own immortality with his twin to keep them together and they were transformed into the Gemini constellation.