One thing we haven't found out in ANY of the seven books is WHY Lady Gisela joined the Neverseen (okay, there are a lot of things we don't know, but...). So here's a CRAZY theory about why she joined.
Lady Gisela had a twin. But unlike Mr. Forkle, who hid the fact that he had a twin, Lady Gisela and her twin had to deal with the elve's prejudices toward multiple births. Of course, Gisela and her twin hated the fact that the multiple births were scorned, so they joined the Black Swan. Remember how Lady Gisela always said that the world had problems? And that the Neverseen were the only ones with an actual solution? Well, if she had joined the Black Swan, she would know all about their plans and know that apparently, her solution was better. But wait! There's more!
Lady Gisela did believe in the Black Swan's ideas. She and her twin worked hard and risked their lives for the order. But then, one day, on a dangerous mission, a tragedy occured. Lady Gisela's twin was injured, and the Black Swan had a choice. They could save the twin, but then they would have to reveal the order to the world. Or, they could let the twin die and maintain their secret identities. They chose to let the twin die.
But Lady Gisela loved her twin. And because they let the twin die, she hated the Black Swan.
She started to come up with a plan to destroy the Black Swan. But before she could take action on it, she was discovered. The Black Swan captured her and wanted to erase her memories. They didn't want her remembering anything, so they tried to actually wanted to shatter her memories. Only... Mr. Forkle kinda had a crush on Gisela. He never acted on it, and she never knew... but he liked her. But he knew that she had betrayed them, so he had to punish her. Unfortunately, nobody was as skilled as Gethen at shattering memories in the Black Swan. If they shattered her memories, her sanity would have shattered too. And Mr. Forkle couldn't bear that. So he convinced the Black Swan to just erase her memories of her twin and the Black Swan and let her go free.
Gisela lived a pretty normal life after that. She got married, etc. And then her memories triggered.
Only some of them... She didn't have the triggers for everything she had forgotten. But she remembered enough. She remembered her twin. She remembered that she hated the Black Swan. But... she also remembered that the world had problems. And that she wanted to fix them. So this time, she joined the Neverseen. The Neverseen had the same goals as the Black Swan, only they were WAY more ruthless. That was fine with Gisela. After she lost her twin, she didn't really care about anybody.
So she joined the Neverseen to kind of destroy the Black Swan AND fix the world's problems.
You know (or don't know because SM hasn't revealed it) the rest.