Nix was a member of the Las Vegas team, and took it very hard when measures against Cauldron capes came into play, or because she had a personal relationship with Pretender, who was detained due to the PRTs new measures.
Nix isn't described in detail, but it can be assumed that her costume is flamboyant, like the rest of her team. She likely resembles Nyx physically, though without her sister's mutated appearance.
Nix can create illusions similar to those produced by her sister, she can control these images, having them move and alter them. However, when these illusions are disrupted they turn into harmless smoke as opposed to the poison gas produced by Nyx. Walking through these iillusions were enough to break them. Other possible powers are unknown, but as she is easily trapped by Golem's stone hand, it can be assumed that she is not a powerful Brute, Changer or Mover. As such she is not a frontline combatent.
It is unknown if her gas has the same properties as Nyxes, while it is not noted to the possibility exists that she is actively repressing the effect. She says that her illusions are permanent, able to function even if she is knocked out or sleeping, but this claim has not been tested.
Nix, and her twin sister Nyx both took Cauldron vials, presumably as part of tests with capes with identical DNA, like Fenja and Menja. Only Nix had viable powers, while Nyx became a "monstrous cape". It appears that Nyx was released into the world as another Case 53, while Nix was allowed to continue her life as normal, presumably joining the Wards and then graduating to the Protectorate.
As part of Vegas Dark she helped with the infiltration of Cauldron after Doormaker was left incapacitated. She helped obscure the Portal to the Cauldron Compound. She was stopped by the strike team the Protectorate sent.
She was last mentioned being taken into Protectorate custody, but given later events it is unknown if she remained in it or even survived.
↑ 1.01.1sanctaphrax: What was the deal with Nix and Nyx, anyway?
Wildbow: Twin sisters, took Cauldron vials, one went Case 53. - Comment byWildbow on Reddit
↑ 2.02.12.2Satyrical gestured, and most of his team joined the L.A. team members. I was left kneeling, still catching my breath. Satyr and Nix hung back, arms folded, exchanging surreptitious glances. [...] “You were going to leave the Protectorate? You had to have been.”
“Yeah.” It was Nix who spoke, not Satyr.
“Just like that?”
Nix shook her head. “It’s gone. Doomed. We lost Alexandria, we lost Legend and Eidolon. The new team doesn’t hit half as hard. Look at Rime. Taken out of action like that. Protectorate’s a shadow of what it was.” [...] “Alexandria worked for them,” I said.
Arbiter looked up at Satyr and Nix, “If you leave, the Endbringers-”
Nix interrupted, “We’ll still fight Endbringers. But the Protectorate was going to take Pretender from us because of how he got his powers. It’s ridiculous.”
“He was still going to be on the team,” Arbiter said. “Just… we can’t let him be leader if he’s beholden to a group like that.”
↑ 3.03.1Every part of the Las Vegas team’s reaction to our arrival screamed dissatisfaction. Folded arms, the way none of them would meet our eyes, the very way they were positioned, so they were just enough in our way to make it clear they didn’t agree with what was going on, but not so close as to be with us. [...] Nix, Blowout, Leonid and Floret joined Satyrical in their anger. Heroes in more flamboyant and colorful costumes than normal, their moods a contrast in how dark they were. Spur and Ravine seemed more lost than angry, but the way they retreated into their group as we passed told me that they would side with their team over us. - Excerpt from Drone 23.2
↑ 4.04.1She stepped into the brightly lit corridors of Cauldron’s testing laboratory and strode down to the cells.
Thirty cells, filled with subjects. Thirty-one now. The cells didn’t appear to have doors, but the individuals within were all too aware of the dangers of stepping beyond the perimeters of their cells, or of trying to harass Alexandria as she strode by.
Only two-thirds of them were monstrous, affected by the formulas. Others would go free with alterations to their memories. Some would have fatal weaknesses inserted into their psyches, reason to hesitate at a crucial moment against a certain foe. - Excerpt from Interlude 15.z
↑Nix Produced gas that could be shaped into controlled but fragile silent images. Protectorate - Parahuman List, bolded edit by Wildbow.
↑“You better hope we make it out okay,” I said. “Tattletale?”
“Pretty sure it’s to your left. Start by going ten paces that way.”
We followed the directions.
The illusion broke, dissolving into harmless smoke, as we reached it and pressed hard enough against the wall in question.
With the barrier gone, I could feel the warm air from within, see a dark hallway without lights. - Excerpt from Venom 29.3
↑ 8.08.1Golem set about binding him to the cave floor with hands of stone.
I looked at Nix. “Her too.”
Golem reached into his costume, and hands of stone gripped Nix.
“To the ceiling,” I decided, at the last second.
“Sure,” Golem said. Hands of stone emerged, passing Nix up. She struggled a bit, but she was at an unsafe height by the time she realized what he was doing.
She was bound to the cave ceiling with armholds, leg holds and an arm set across her collarbone. - Excerpt from Venom 29.3
↑“Where’s the portal?” I asked Spur, ignoring Imp.
“No portal. Or weren’t you paying attention?”
I looked at Nix. “You know where this goes, if you don’t cooperate. Circumstances are a little too dire. We knock you out, your power fades. So why don’t you drop the illusion and let us see the portal?”
↑Her eyes tracked the rows and columns of incubation chambers. They weren’t her field either. A different row for each member of the Slaughterhouse Nine, past or present.
King Screamer Harbinger Breed Crimson Gray Boy Nyx Psychosoma
There were ten of each in various glass chambers. The original members. - Excerpt from Interlude 25
↑Cauldron's habit was to take all of the human experiments that turned out well, brainwash them, and then place them in larger organizations to support said organizations, fighting the natural tendency for parahumans to seek conflict (and thus making forming large committed groups hard). Shamrock could well have been slated for the Protectorate, Red Gauntlet, the Suits, or even the Nemesis program (being brainwashed with an auto-lose trigger against one client who paid a good sum, so that client could get a better position and climb faster in rep). - Comment byWildbow on Spacebattles
↑The Protectorate is just on the other side of the portal, collecting Satyr’s teammates, Nix and Spur. - Excerpt from Interlude 29
↑Blunck, Jürgen, Solar System Moons: Discovery and Mythology (2009), p. 129.