History and Mythology*:
The constellation Gemini is said to represent the twin sons of the Spartan Queen Leda. Pollux was the son of Zeus and was immortal while mortal Castor was the son of Leda's Husband Tyndareus. The twins were very close and when Castor was killed in a battle with Idas and Lynceus, Pollux begged his father Zeus not to separate him from his brother for eternity. Zeus granted the request and Castor and Pollux are said to spent half of their time in the heavens among the gods and the other half in the underworld; alternating daily.
Characteristics of those born under the Air sign of Gemini*:
articulate, intellectual, charming, witty, joyful, adventurous, spontaneous, imaginative
*Note: Information in this description on the history and astrology of the constellation Gemini was drawn from the Wikipedia article.