One thing that I think acosf kept consistent was that in previous books Nesta made a difference in everything. She was significant even when she was not the main character or had an active role in any decision making.
Feyre in the first book references Nesta more than anyone. Albeit not for good reasons, but when she learns that Nesta tries to go after her, that makes a significant difference in Feyre’s next move. She goes back and continues with plot because of what Nesta says. She shifts the narrative. Outside of this, she’s also singled out by being the one’s who favored by their mother, the one who can withstand glamour, and the one who chooses to risk her life for Feyre if she’d actually gotten past the wall. She’s the person who is “made different.”
In acomaf, the person who immediately goads at Cassian, who is referenced to have some future altering affect on the relationship between Mor, Cassian, and Azriel which existed for years is just the introduction of Nesta. Just her complete devotion to keeping the humans safe is enough for Cassian to lay his life to her cause. Later, Nesta is not the only one to be dipped into the cauldron, but she’s the only one who takes. The only one who’s vendetta, her risen middle finger, scares the King of Hybern. She makes two enemies that day. She again shifts every plot point later, because of this power and experience.
In acowar, she’s the one who literally does nothing and we have Amren who says “when you erupt, make sure it’s felt across worlds.” The Bone Caver speaks of her, of the imminent power. Just the mention of an insult from Rhys, is enough to make an argument between Feysand. Later, Nesta saves them all. If not by telling her story to the high lords, but by being the distraction. That’s her plan. She doesn’t stab the King of Hybern, but Hybern would not be dead at all, there would be no opportunity to even get that close, if Nesta had not decided to tell them to use her as a distraction, knowing full well that it was some sort of sacrifice.
In acofas then, her depression irritates them. She’s in like 10 pages of the whole book and yet she is the person they discuss over and over again, because her attitude affects them all. They can’t leave her alone because none of them can be completely happy, when Nesta is not happy. That’s her power. Nesta makes Elain, and Feyre, and Cassian upset and unsatisfied which means that anyone related to them like Mor or Rhys or Amren or even Azriel are also not satisfied. Their group also functions with compliance, so no matter what she is the deviant. She is the singular.
So in acosf, this continues, because literally all Nesta does is start training and she convinces priestesses who don’t leave the Library to come train with her. She convinces Emerie, the first willing Illyrian female to fight. She is the reason Gwyn leaves for even a moment. She helps the IC once, not even because she really wants to, and she’s not using very much power and yet she begins to find three of the most dangerous items in Prythrian and then makes three more. She went to the prison and just killed a death god when Cassian’s fully certain he’s probably going to die. She is the reason why her friends make it to the top to Ramiel. They would not have without her. Even in terms of like minutia of everyday life, Cassian knows this blacksmith all of these years and only Nesta gets him to be a little softer. Only because of Nesta, do we become aware of a Illyrian male who goes against the norm. Eris immediately knows that Nesta is the true prize as much as Lanthys and the Bone Carver. Nesta is the only one Mother Blessed. The only who has and can stop time and who saved all of their lives once again.
So in conclusion, Nesta is literally the main character of this whole series. She plays the Aelin or the Bryce. She is the “chosen” one. It’s not technically anyone else. They may get a pov, they make steer plot, and we may like them a lot and their romance, but the person who plays that typical role of the hero arc and who influences the story in its entirety is Nesta Archeron.