Song is Alluring secret, black vow. It's about Rin, an angel, who falls in love with Miku, a human. She gives a wing to become Len. Miku was already going to marry someone, but dated Len/Rin instead. The person Miku was supposed to marry killed her, and Rin gave another wing to save Miku, but because Rin gave up both wings, she vanished, and left a single feather.
