The Dark Card is somewhat of the best friend of Light or something. They appear together (like Rain and Wood), only her powers are to engulf everything in a black sheet of darkness. Her appearence is very similar to Light, only her dominant colors are black.
Dark appears in Episode 42 during the Sleeping Beauty play. She causes everything to be plunged in darkness until Light appears from within Sakura's heart. They are reunited, and they warn Sakura of Yue. When summoned as a card, Dark will unleash her powers of darkness.
