- "Vanessa Santoro may not be perfect, but she knows how to keep a secret."
- —Vanessa in Keys to the Demon Prison
Vanessa was a former member of the Knights of the Dawn, and an also former agent of the Spinx. She eventually turns against the Sphinx and fights alongside the Sorensons. Her love interest is Warren. Vanessa is a narcoblix, a species who can control people's bodies in their sleep.
Vanessa Santoro first appears at Kendra and Seth Sorenson's home. She had been sent to pick them up and take them to Fablehaven, away from Errol, who claims to be an ally, but ends up being a member of the Society of the Evening Star.
At Fablehaven, she is presented as a keeper of magical creatures, and slowly gains trust from her comrades. Vanessa establishes trust with Kendra, saying she's her "pen-pal.' She gives her an umite candle so they can write each other secret messages.
Later, when exposed, turns out to be a Narcoblix. Vanessa was actually a narcoblix, a blix who can control bitten victims in their sleep. And she finally reveals herself as a traitor, and a member of the Society of the Evening Star. She overthrows the house and throws most of the Sorensons and their allies in the dungeon, Kendra and Seth excepted.
Vanessa temporarily joins forces with Kendra and Warren in the Inverted Tower in order to defeat the Guardian Cat and secure the artifact another. During the fight, Errol gets killed and she gets coated with it acid, though she is saved by Kendra. After she admits she should be punished, she is imprisoned in the Quiet Box in the Dungeon of Fablehaven so that she cannot control anyone whom she has bitten. Before she is put into the Quiet Box she leaves a message for Kendra, telling her more about the Sphinx.
In the third book, she is released for short periods of time to be pressed for information. Vanessa conceals many things, but offers her insights of the plague's origins. She hints that she still has a secret yet to be revealed.
In the fourth book, she is freed by a Stingbulb copy of Maddox Fisk, but chooses to help the Fablehaven caretakers capture the imposter and press him for information at the same time giving up several opportunities to rejoin the Society, return critical information to the Sphinx, although she later hates the Sphinx. She is given a cell phone and is allowed to live in a cell with some efforts to make her comfortable while she uses her powers to take control of Society members she's bitten to gain information. However, this freedom comes with a warning. If she takes control of Stan, Ruth, Tanu, Coulter, Seth, Warren, Dale, or Kendra (whom she could never control in the first place) they would take it as a clear sign of where her loyalties lie and kill her. She does however break this rule and takes possession of Tanu, saving the expedition at Wyrmroost from Glommus, but given the circumstances, her intentions and the results of doing so, it's overlooked, though they still remain wary of her. Vanessa also reveals some of the inner workings of the Society, including how the Sphinx was always its leader, and claims to hold a secret so great it would likely buy her her freedom and she cannot reveal it just yet.
Vanessa helps Seth fulfill his plan he received from Patton Burgess. When Seth is discovered to have asked wraiths to attack centaurs, he is attacked by the creatures. Vanessa manages to saves Seth, Newel, and Doren with her blowgun, putting the centaurs to sleep while biting all of them.
The next morning, Vanessa drives the satyrs, Hugo, and Seth to the location of the Singing Sisters after being told by Patton Burgess. They reach the location, greeted by a man. After conversing for a few moments, the man accepts Seth's cause to bargain with the Singing Sisters, and agrees to escort the group to their island provided they leave their weapons behind. After Seth lands a successful deal with the Singing Sisters, the group goes to the Totem Wall, so Seth can obtain Vasilis, a powerful sword.
After Seth lays hands on Vasilis, the group goes to find Lady Luck, a boat that will take them to where Zzyzx is located. The group encounters Bracken, Kendra, and Warren. The dislike between blixes and unicorns is revealed here, with Bracken trying to access Vanessa's mind, telling the group that the narcoblix had strong feelings for The Sphinx. Vanessa counters the unicorn, revealing that Bracken held feelings for Kendra. Trask breaks this argument.
She also assists in the Battle of Zzyzx. Warren Burgess is revealed to be an assumed love interest. She's around when Zzyzx opens. It is also hinted by Bracken that she used to be in love with the Sphinx in Keys to the Demon Prison.
Vanessa is briefly mentioned in the start of the book in the chapter Danger. She loses her sight fighting the first guardian by looking at Kendra's fairy ring.
Vanessa is described to look Spanish or Italian. She has dark lustrous hair and an olive complexion. She appears to be tall, slender, and stylish. Vanessa is shown to wear expertly applied makeup and trendy clothes.
Vanessa is shrewd, cunning, and patient. She is shown to be remarkably resillient, shown when she survives many perilous missions, and even when she endured a stay at the Quiet Box. Vanessa is shown to be genuinely kind and ready to help. She is undyingly loyal to those who are faithful to her. Vanessa is very brave and resourceful, and despite her history, is a good person.