Nesta loves her sisters more than anything. Otherwise she dislikes pretty much everyone. She has a tough time with people, communicating and socializing with them. For some reason she gets along well with Elaine, but Feyre brings out her awkwardness and hostility even more than usual. For some reason everything the one does the other sees as in the worst light, and despite both loving each other they have never gotten along.
Nesta sent her sister into the forest to hunt because to her, being alone in the woods was absolute joy, something she would have wanted for herself more than anything, and she wanted her sister to live that. She took upon herself the burden of managing her household and their father.
When Feyre left, Nesta hired that mercenary to track her. Alone with that strong capable woman in the woods was the happiest she's ever been up to that point. Nesta was more than half in love with the mercenary, but would never admit it, even to herself. By this point, she internalized the idea that she is unloving and unlovable. Her demeanor was companionable at best.
So then events of the ACOWAR happen. At the end of the story Nesta is living in Feyre's house, or more specifically Rhys's house. She does not trust the Fae, does not trust their power and their magic, including mating bonds. She also has no idea what her magical power is or how to use her it. So she's living with all these people she doesn't like or trust, and wants to figure out her own power and establish herself on her own terms. Ultimately she wants to protect herself and her sisters from the overwhelming power that Rhysand and his friends wield. Feyre doesn't know this and would be enraged if she found out. Also Cassian is always trying to provoke her, and she finds him deeply annoying but a little attractive.
First she starts at the library, learning about the history of this land and the theory and practice of magic. She is also figuring out her power, which allows her to sense and manipulate lifeforce in all living things. She learns she has an affinity for plants and animals.
Nesta makes plans to go into the deep forest. She doesn't want to leave her sisters but knows that they will be safe for at least a little while. Once there, without all the Fae and other faeries, she can finally breathe freely. She follows her power into the deep forest where the lifeblood of the trees beats like a heart.
She meets Leshiy, the guardian of the forest who tries to drive her away with a storm of rain and wind and mud. Of course, while Nesta was hesitant and uncertain about her course of action before, now that someone opposes her, she stubbornly determines to stay no matter what.
Nesta meets the dryads of the forest trees who become her earliest allies. She learns how to call to dying animals to come to her so she can harvest them to eat and maintain balance of life and death. She can sense the birth, life, and death of any living thing in the forest, from the tiniest mite to the largest were-bear, from a blade of grass to the tallest tree. She learns that she can take life from one being to give to another, and how to do so without destroying both.
Meanwhile, Leshiy comes in many forms to either lure Nesta into a trap or to drive her out of the forest. Sometimes a stranger, or a friend, or an animal appear like a mirage among the leaves, just out of sight. Sometimes her shelter crumbles or a mossy riverbank suddenly floods. With each confrontation she finds new uses for her power and persists.
For a year they do this, and as time goes on, it becomes more of a game, a dance. Neither one wants to cause any harm to the forest, and Nesta's magic and her control over it grows stronger. The challenges that start out destructive become more clever and creative as the two learn from each other and develop a companionship. Sometimes the pair are playful and flirty, but neither one is really sure. A misanthrope and a personification of woodlands have no ability or practice in flirting and when they try only awkwardness ensues.
Although the heart of the forest is far from Fae society, rumors of a new powerful witch reach the Courts. Feyre puts two and two together and figures out that her sister is behind the rumors. She asks for Nesta to come back.
Nesta has her own reasons for returning - the longer she stays away, she worries, the more her sisters are enmeshed in the Night Court. She wants to see them for herself and make sure they are well. She also sets herself a task - she wants to gently signal that Feyre and Elain can turn to her if they need support or help that's not dependent on Rhysand and his court. The problem is that Nesta doesn't do gentle or subtle, especially where Feyre is involved. She spends her travel time coming up with a plan to get her sisters alone together, and hopes that it doesn't backfire and lead to more conflict.
She comes back to the city and her senses are immediately overwhelmed by the crowds of Fae and other creatures and animals. Only her purpose keeps her from running away back into the wilderness. Returning to Court, she is met with varying degrees of wariness and hostility. Cassian tries to keep up a flirtatious banter but the threat assessment underneath is obvious, even to someone as oblivious to non-verbal cues as Nesta usually is. Elain is the only one whose welcome is not tinged with suspicion.
Barely keeping a lid on her irritation, Nesta convinces her sisters to come with her to their old cottage, where she tries to forge a magical bond of sisterhood that, if worst comes to worst, will allow the sisters to stand together. As expected, Feyre takes this as a direct challenge to herself and Rhysand. She banishes Nesta back to the forest.