Nerissa is calm and calculating, and she will step on or over anybody in order to further her cause. She even gave birth to a son so that he could overcome her enemies and lay down the foundations for her rise to power. She has an unshakable belief that it is her right to take control of the universe and to reshape it in her own image.
Despite her calm exterior she is prone to fits of emotion (as shown when her son Caleb refused to join with her) and, although she does not show it, Nerissa is consumed by feelings of guilt over Cassidy's death and in one episode when Yan Lin says to her "Are you going to destroy us like you destroyed her, does her memory mean so little to you?" Nerissa responds "You have no idea what she meant to me" before screaming "This is not over" and fleeing, leaving Yan Lin and the Guardians alive even though she could quite easily have destroyed them.
Nerissa is also a very skilled actress, as she was able to fool everyone, including the Oracle that she was the Mage and Trill without giving away suspicion, until she exposed herself. She was also able to fool Kadma and Will when she was running away from them in order to get Kadma to absorb the Heart of Meridian into the Heart of Zamballa, and fool the Guardians that she did not want them to rescue Yan Lin. It is obvious from that she also has a lot of patience, as she was forced to wait approximately 14 years to get the Heart of Meridian.
The previous keeper of the Heart of Kandrakar, Nerissa was consumed by the immense power the Heart gave her and eventually began to descend into evil. The Oracle sensed this and decided that she was no longer fit to keep the Heart. When he entrusted it to Cassidy (who was then the Guardian of Water), it drove Nerissa over the edge. Mad with jealousy, she turned on her fellow Guardians and killed Cassidy. When she was defeated, she was sentenced to be sealed in a coffin located in Mount Thanos until the day all of the Aurameres were joined together. That eventually came to pass through the efforts of Luba, who was convinced that the girls were unworthy of their status as Guardians.
Awakened from her slumber, Nerissa got her teenage looks back, created the Four Knights of Revenge, and tricked Will into giving her the Heart, declaring war against Kandrakar. After losing strength in the battle with the Guardians in Kandrakar, Nerissa re-absorbed the magic she used to create her Knights of Revenge, destroying them automatically. In the end, the Guardians combined their powers and directed them at Nerissa at once, vanquishing her for once and for all. In order to perform her evil plans, she created Shagon the Hate, Khor the destroyer, Ember the pain and Tridart the dispair.
One of Nerissa's more insidious powers is her ability to change the girls' dreams, turning them into nightmares. The dreams may occasionally affect the real world as well. At the beginning of the second arc, Will's arm is branded with Nerissa's special seal (shaped like the Heart of Kandrakar, but with one scar-like line crossing it) during a dream. It is implied that Hay Lin is less susceptible to these fiendish assaults, since her grandmother, Yan Lin, was Nerissa's best friend before their group was dispersed. One sequence sees the team confronting their tormentor in the so-called "World of Nightmares", entering a "mutual dream" by going to sleep in the same room. This is another episode which stresses the importance of unity – the girls manage to survive the dream because they are together, as opposed to being attacked one by one. They succeed in trapping Nerissa in the Heart, and then wake up. However, they find she was only defeated in the realm of dreams, and that they still had to face her in reality.
As the former holder of the Heart of Kandrakar, Nerissa has all of the same powers as the current Guardian Will, although she is much more experienced in using them, and much more aware of their true potential, than her younger counterpart.
Her elemental power is Quintessence, which can bestow life on inanimate objects. It can also be used like conventional electricity, allowing her to attack an opponent using powerful lightning bolts and to form an electrical shield around herself. Nerissa can even create new life, as she did in order to create Ember and Tridart. Nerissa's ability to use Quintessence is so advanced that she is able to raise the dead (As seen when she raised the spirit of former Guardian Cassidy, which she then bound to the mortal world using the Heart of Meridian). She is also an illusionist able to change her own appearance and to twist the minds of others to her will, however because she isn't actually changing the shape of her body, her shadow can still be seen in her true form. This allowed the girls to find out who she was when she transformed into the Oracle, trying to get the Heart off of Will in "I is for Illusion".
Nerissa also possesses a terrifying ability. She can manipulate the dreams of others. She displayes this ability in the episode "E is for Enemy" when she attacks the Guardians in their dreams. She even causes these dreams to manifest in real life, drenching Cornelia with water and taking most of Irma's voice away. When the Guardians faced her in a "mutual dream" they were able to defeat her and create a psychic backlash, which knocked her from the dream plane.
She also seems to possess the power of Telekinesis, as during her time as posing as the Mage she was able to levitate the Tonga Tooth that she had taken from Blunk claiming that there was a spell on it. She has a high knowedge of spells, as she was able to place a spell on Blunk's Tonga Tooth enabling her to track the folding movements, and knowing spells to control the minds of other (as she did with the other members of CH.Y.K.N.), and presumidly many other whilst posing as the Mage,.
Nerissa supplemented her powers using the Heart of Meridian, which she stole from Elyon, allowing her to teleport freely, even between dimensions, and greatly enhancing all of her existing powers. Later, Nerissa tricks Kadma the former Guardian of Earth, into merging the Heart of Meridian and the Heart of Zamballa, and takes control of both Hearts, fusing them together to become the Seal of Nerissa. Further enhancing her powers in all respects, and raising her control of Quintessence to the level where she was able to turn Cassidy's spirit into flesh. Nerissa later gains control over all the elements when she absorbs her former friends into her Seal.