Serinth is a tricky character to write.
Through most of Series A she's portrayed with an almost psychotic indifference to the well being of her pack, leading the audience to wonder why they're even following her to begin with, though as we see in their backstory flashbacks they all have their own reasons. She seems emotionally numb, constantly brooding and bored, as if always the person least interested in what she's saying. And she rarely talks, rarely even looks at the people who are talking to her.
This is all in contrast to Series B where we see as a young woman she had a completely different personality. Furthermore, she seems to suffer from random mood-swings that border on mental illness, sometimes makings comments regarding events that never happened, leading her allies to consider whether she's succumbing to the Rot. When writing her dialogue in Series A, I want to leave several hints as to her true nature, many of the things she says that seem wrong or ambiguous should make complete sense upon re-reading the story. It also makes her one of the trickiest characters to write.
There are three factors to keep in mind when writing her scenes, and all three are caused by the events that happened at the end of the Battle of Tairon:
As we see in Series B: Cubs in Zarkhan, like Heyoke and Triavox, Serinth is an esper. She still has some of the latent gifts of the older bloodlines, but unlike Heyoke who can reave, and Triavox who and limited seer abilities, Serinth's is an empath. She has the ability, with practice, to feel the emotions of those around her, especially if she's looking at them. Of course it's taken years of training to practice, and works best when she's staring in the eyes of the other person. As she peers into their feelings, they also see fragments of hers, and the experience can be uncomfortable.
After the Battle of Tairon, this ability is broken and 'always on', causing her great chronic pain as the emotions of those around her constantly flood into her, causing her to seek seclusion whenever possible. When people die around her, which we eventually see happens constantly, she feels their pain flood into her. And since this esper ability is always on, it makes people incredibly uncomfortable to face her eye to eye, as it feels like she's staring into her soul. So she normally doesn't maintain eye contact during her conversations.
The Endless Loop
As another side-effect of the events of Tairon's final battle, Serinth has gained practical immortality as she has the ability to jump back in time. More specifically, her empath abilities now not only extend to those around her, but also alternate versions of herself through the Nether. Serinth is 'stuck between worlds', living multiple lives at once.
If she gets killed, she does die, but a different version of her in the recent past still has her memories, so from the point of view of her 'conciousness' she can never die, even when she tries to kill herself. Just like Triavox 'jumps back' after seeing the future, she can 'jump back' through her past, by switching to versions of her in alternate (but almost entirely identical) timelines, which is why it seems like she never seems to get hurt. However she can never jump back to before she wakes up after the Battle of Tairon, she's always stuck in the legacy of the genocide she helped commit.
And, as we see near the end of Series A in her flashback, she's already lived most of the events of Heyoke the Wolf before. She's already killed her father a dozen times, taken his place as the leader of the republic, formed armies to combat the Deadwood. But as she explains to Jagex, it doesn't matter, the Deadwood always kills them all. Eventually she gave up hope completely, instead spending years reliving her daughter's birthday until during one loop Jagex noticed and convinced her to jump back to the beginning again (after she wakes up in the aftermath of the Battle of Tairon).
Heyoke the Wolf (book)
After having given up before, and still trapped a personal hell of her own making, she realizes that not everything resets when she jumps back. The Deadwood has always seemed to defy her abilities, as if existing independently of the rest of reality, she can't navigate to her other selves within it. But Triavox also, who after the Battle of Tairon is nearly comatose, and constantly repeating random bits from Rosewater, says something different each time she jumps back. She realizes there is something special about Triavox (which there is, because just like her he's been knocked into the Nether).
While writing down his ramblings (which are repeats of the Rosewater transmissions), he begins narrating the first volume of Heyoke the Wolf. He's narrating the book that Serinth is in, and as she writes down what he's saying, he gets to the part describing her writing down what he's saying, including her thoughts, terrifying her. But she continues to write, and when he finishes she realizes that she has the complete outline of what happens next, all of the first volume's text, which she is paradoxically apart of. As she goes back and reads her notes, she gets to the part where the book describes her reading the notes, and she realizes as (she's thinking it and reading about her thinking it at the same time) that if she can follow the book it will lead her to the way out of the loop.
Which is why she's constantly trying to navigate the Deadwood, recording each time they die, which ends up filling an entire library, because she's been doing it thousands of times. She knows that she needs to continue repeating events, repeating the same dialogue, until she gets to certain milestones in the book. She knows she needs to fill up the library until Pyren comes in one day snooping around. She knows Shuik needs to die to the moose when they first get to Deer Park, or she needs to revert and try again.
This is why from everyone else's perspective she seems to have locked herself in her library for the last nine months, her personality slowly deteriorating as she writes in her 'memoirs'. From an outside perspective she seems to be scribbling down a paragraph or two in her memoirs, pausing for 10-30 seconds as if thinking, only to scribble down another paragraph. From her perspective she's writing down what went wrong, leaving with her pack to the Deadwood, dying with everyone else, jumping back to the journal, and writing down the next entry. It's also why she seems to randomly cry and laugh for no reason, she's crying because of what happened right before she jumped back, she's laughing because she's so excited that something different has happened for a change, something she knew should have happened for the story to progress. It's also why she seems to be so constantly frustrated and bored and aloof all the time, and explains much of her odd dialogue.
For instance, when Heyoke asks her what she's reading in Deer Park, she just bats an eye to him and he apologizes for asking, and she replies that she doesn't want to "limit her choices", because if she told him what she was reading, since it's part of the story, she would have to read the same book over and over again for the story to progress (he's already aware she's stuck in the loop). But that doesn't mean she can't do things differently, as she explains to Pyren (who thinks she's insane), as long as she sticks to the script she can 'write her own story in the margins'. Since she knows exactly what happens in Heyoke the Wolf, as long as she's not participating in one of the scenes mentioned in the book (that the audience is reading), she's free to change the outside narrative as she desires.
So when we see her first in Deer Park, she's been repeating her trek into the Deadwood, and repeating the same tired dialogue, for what might have been nearly a century. She's been doing it for so long she can't even remember her life before the Battle of Tairon.
Her personality shifts several times in the story, we see one shift in Jagex's flashback where she goes from seemingly full of confidence (after getting the book from Triavox and having a plan to stop the loops) to dark and cynical as she locks herself in the library (and constantly fails to traverse the Deadwood).
We see in the first third of Into the Deadwood that Serinth seems emotionally void even as her closest allies die around here. In Cubs in Zarkhan we also see the young Serinth act cold to her new brother as she quickly becomes envious of his abilities. In Deer Park's last scene we see Serinth call Heyoke 'mutt' and it seems like an insult in the context of the conversation, the two characters don't talk very much until they regroup in Marathon.
However by that point the reader has started Cubs in Zarkhan and sees the pair bond as children over time. At the end of Cubs in Zarkhan Serinth tells Heyoke that she's going to protect him, and if he wants to keep calling himself 'Muts' then she's going to call him a mutt, and as she continues calling Heyoke 'mutt' in the present we realize that this isn't an insult but a rare display of intimacy in Serinth's otherwise cynical outlook, one she only reserves for those closest to her.
By the time Serinth returns to Weston in Series E and talks to Heyoke in Mosphet's backyard graveyard, we see how close they really are, that they tell each other everything, that he is the only person she really trusts.
They don't need to talk to each other during the Deadwood arc, they already know why they're there, they've known each other for fifty years, like an old married couple they have their own private language.
We really start to see their relationship in full in Series E after Serinth hugs him while he cries over Mosphet's body, how from that point forward they begin to rule together. He's not a subordinate, he never was, she sees him as an equal.