Sakiko Hiou (Vampire Knight)

It has always been thought that Shizuka Hiou was the last Pureblood of the Hiou Clan, but there is one more. Sakiko Hiou, Shizuka's twin sister. Sakiko was in a three thousand year old slumber, being awoken the day before Shizuka attacked the Kiryuu family. Sakiko takes Zero Kiryuu under her wing after Shizuka turned him into a vampire (and after Zero tried to kill her after mistaking her for Shizuka). Four years later Sakiko and Zero transfer to Cross Academy's Day Class, Sakiko pretending to be Akira Kiryuu, Zero's younger sister. No one knows that Akira's true identity is one of the last remaining Hiou Purebloods, Kaname Kuran, the last Pureblood of the Kuran Clan (for the time being) being the only one who knows her true identity.