Helena is a more delicate and dainty version of Helen, a favorite of Shakespeare, who used it in both All's Well That Ends Well and A Midsummer's Night Dream. Historically, Helena was the mother of Constantine the Great (and, supposedly, the daughter of Old King Cole), who became a fourth century saint.
British star Helena Bonham Carter and Danish model Helena Christensen have done much to keep the name alive, and it is also remembered via beauty mogul Helena Rubinstein. Helena is also the title of a song by the group My Chemical Romance.
Helena is a recommended choice for those who find the classic version a bit bland.
HELL-ehn-a is the best modern pronunciation for Helena, but hell-AYN-a or even hell-EEN-a work too.