Vespera, Lady Vespera, or The Forgotten Prisoner as called by Sophie. She was locked in Lumenaria for an unknown amount of time for an unknown reason (it's hidden in the Councillor's caches), until Fintan Pyren, Gethen, and Ruy Ignis broke her out (Brant was there, but was killed by fallen debris). She created the original Nightfall, which Lady Gisela based her version of Nightfall off of.
She also was the one who (according to Councillor Bronte after translating the journals Lady Gisela provided) caused the skirmish between elves and humans, not because humans were greedy, but because she was abducting hundreds of humans in Atlantis and the old Councillors did not pay any attention to the humans' reports, complaints, and pleas.
She is an Empath. Keefe speculated that she felt too much with her empath powers and went numb, not caring about anything anymore. This explains how she is able to torture humans when elves are supposed to go catatonic or have their mind broken.
In the end of Book 6, Nightfall, she joins forces with Lady Gisela because it was the best option over imprisonment, though still viewing Lady Gisela's views incorrect and sees her as a person who took her dream (Nightfall) and did not see the point of it and built a flawed version of it. When Sophie and her friends went to her nightfall to rescue Sophie's parents, she showed herself in an illusion of mirrors and light. She and Biana fought against when Biana sneaked into a secret place where Fintan Pyren and Vespera was hiding, and she smashed her into the mirror. Biana got a light permanent scar in her face.
Vespera, of course, like all elves, is very beautiful. She is seen to be a shockingly pale female. Her features included thick arched eyebrows, angled cheekbones, long pointy ears, and a sharply curved mouth. As said in the book, Vespera has a beautiful gown which used to be fitted, but now hung off her shoulders due to weight loss.
Vespera is quite cruel, unkind, and always sees the people around her who work with her as unneeded and could be traitors. This is shown in Atlantis as she built her own facility underneath it, and its purpose was human experimentation. She kidnapped hundreds of humans who lived in Atlantis and experimented on them, trying to see why their minds did not shatter when they committed acts of violence. As well as experimentation, it is possible that she kept them down there until they died, experimenting until the very end as letting the humans go would possibly get her imprisoned. Another example of this is when Sophie is in her version of Nightfall, she says that every person around you should be disposed of because it means one more person to ruin your plans and betray you. She also says that Sophie is weak because she tries to save everyone, while she should just escape with not everyone but some people.