Nerissa is the secondary Antagonist of W.I.T.C.H.
Nerissa was once the holder of the Heart of Kandrakar and the Guardian of Quintessence, but it was taken from her because she was corrupted by its power. Mad with jealousy, she turned on her fellow Guardians and accidentally killed Cassidy by shooting her off a cliff with lightning during a fight for the Heart of Kandrakar. For this crime, she was sentenced to be sealed in Mount Thanos until she was freed by a portal (in the comics the occurrence of all elements combined in one evanescent being). She went trough the portal to Meridian where she met the Mage, who had hoped that she would reform if shown compassion.