Civilight Eterna is an incredibly incredibly interesting character, and has so so much going on so I thought I'd share my read on her.
First off, Civilight Eterna isn't Theresa. Like, at all. In her dialogue, she repeatedly says she's not Theresa, talks about Theresa as a person distinct from her, and seems uncomfortable when the Doctor treats her as if she is Theresa.
The way I see it, she is actually far more like Kal'tsit and Amiya than she is Theresa. Once you accept that, I think Civilight Eterna becomes an incredibly, incredibly interesting character who provides a large amount of great characterization to Theresa.
The real Theresa is dead and gone and erased forever now, but ironically, if anything, after ep14, the narrative is far more haunted by Theresa than it ever was. For the most part, no one is particularly happy with CE being on the landship as she is: for Kal'tsit, who had finally come to terms with her grief and loss, all CE really does is dredge up all the pain and sorrow and hatred she felt about Theresa's death.
For the Doctor, she is a source of self-doubt and guilt over their past sins.
There's also an interesting contrast between her and the Doctor, as CE spells out, with one of them being an existence entirely of memory, and the other having none at all.
Amiya is mostly ok about her, but even for her, CE a reminder of who and what she as lost. She sees CE and understands and sees her not as Theresa, but as something else, but because of her resemblance to Theresa, she can't help but be reminded that Theresa is gone, and she won't be able to see anything she's done or gone through anymore.
Theresa, herself, is gone for good now, but the shadow of her last deed hangs over them all. CE as a character takes the enormous background presence Theresa has in the narrative and she makes it a lot more personal than it already is. As a character, CE takes the ghost of Theresa that's been haunting the narrative in the background and drags it kicking and screaming into the forefront, forcing the main trio to confront it.
The person most haunted by Theresa is Civilight Eterna herself. She was made with the purpose being a replica or substitute of Theresa, with the memories of Theresa, but she's really not her, and she's in fact quite bothered by such a concept. The impression I get from her files is that her arc as a character (provided they give her continued story content and don't just throw her by the wayside) is mostly about her struggles to find an identity for herself beyond the one she was assigned and was made to be (which very much parallels much of Amiya's struggles throughout the her entire character arc!)
In most of the story, Theresa is framed as this, almost mythical, larger-than-life figure, who you hear of often and her legacy greatly influences everything that unfolds, without actually appearing except in brief glimpses and flashbacks (until the story starts peeling back the curtain on her with Babel.) For someone as existent and present as CE is, there is no way that she could actually live up to the legendary figure that Theresa was, nor does she want to. The mythos surrounding Theresa is so great that just by virtue of being made to be like Theresa, this serves to erase any identity or personhood of her own she might have (once again, parallels with Amiya.)
(This is also somewhat painfully ironic on a meta-level. If you engage with CE's content at all, it spells out the fact that she is very much not Theresa, but just because she was made to resemble her, even the playerbase of the game thinks of and sees her as Theresa!).
The thing that's juiciest about this to me is that the relationship she has with Kal'tsit. Kal'tsit is maybe the one person who can truly empathize with CE and her mode of existence: both were made and instilled with some driving, defining purpose, but wish to carve out an identity of their own; both occupy some messy, intermediate state between humanity and inhumanity; and both struggle with the risk of being subsumed by some ideal or identity far beyond what they could possibly be. But instead, Kal'tsit can't bear to see the sight of CE, she gives her the cold shoulder, and is unwilling to meet her eyes. Even if Kal'tsit does understand that CE isn't really Theresa, she can't help but see her and think of Theresa, and for CE, this is incredibly, incredibly alienating, that the one person who should be able to truly understand her and her struggles to find an identity for herself, sees her more as the person she was made in the image of and could never really be, instead of her own person.
In light of the fact that CE's files state Kal'tsit has been avoiding CE is so fucked actually, especially when you think about this in the context of Lone Trail. Lone Trail takes place over an (in-setting) year after CE shows up, and despite this, she is never been mentioned or referenced at all, even obliquely. Has Kal'tsit just been coping about it for an entire year straight*? Like, here's about how I imagine this goes:
The Doctor mentions CE to Kal'tsit exactly once and immediately gets the hint to not do so. They don't get a lecture about this. They don't even get a Kal'tsit Stinkface Talksprite. She just looks, however minutely, a slight bit more tired and sad. And so that's the last time they ever bring it up.
And the thing is, CE has the same care and love that Theresa had, and she wants so so desperately for the people Theresa (and by extension her) cares about to be happy. And so if the only thing she can do for Kal'tsit is to remain completely out of Kal'tsit's sight, existence unacknowledged, no matter how much she might want to reach out, no matter how soulcrushing and painful it might be to do so, she would do this and bear all the pain, out of the love she has for Kal'tsit.
The main way we see Theresa characterized before Babel and Ep14 is through the things and people she's left behind:
the people who lived because she died, a dream, a photo of her snuck when she wasn't looking (she was looking), a flower garden, a letter, a promise, a girl made to bear the weight of the world and an apology to her. CE is the last thing she leaves behind, her last act, her last sin before she exits the story for good. In particular, I think CE as a character is some really good final characterization for Theresa, one that serves to highlight her character flaws.
As a person, Theresa is sort of messy. I mean this specifically in the sense she is a person who does great and grand things, knowing that there will be consequences (both foreseen and unforeseen), and does them anyway, often leaving a mess in the process for others to pick up. In Warfarin's 2nd operator record: her attitude towards Theresa highlights this well, to her, no matter how noble her ideals and how good her intentions might be, the fact of the matter is that she is starting a civil war who which will get people killed. In this regard, I think her creation of CE fits her character perfectly. What she does in episode 14: finally being able to free the Sarkaz from the shackles of Originium, and making CE as a final act of care to Amiya, fulfilling her promise to Amiya to always be with her, but making a huge mess in the process, serves as a perfect capstone to Theresa as a character. Even though she is now gone, the effects of her last choice and legacy still hang over those who knew her.
*Of course it could also be that they wrote LT at a time where there were no plans to have CE be a character at all, but especially when it comes to things like this, HG does generally write things years ahead of time. The ending of Lone Trail does feel very much like it was written with Babel already in mind, and Vigilo was also clearly written with Babel in mind (see: Memoria specifically) despite being released several years before Babel.
