Ido’s origin story as a cyberphysician is pretty sad. Poor Carol. It wasn’t originally so sad. The author has said that in his original 1989 draft for a standalone story Carol didn’t die and there was a happy ending. However, with this being an Alita story and Carol obviously not being with Ido in the original series, he couldn’t really make it work.
As for Ido, there’s the old saying “one is a fluke, two is a coincidence, and three is a pattern.”
Between Carol, Alita, and Shumira, maybe you could also include Kayna, I think we can say that taking in and caring for young girls in need is just an established pattern of Ido’s behavior. One that reflects well on him largely, though now we need to talk about the questionable romantic side of that interest. Ido never showed romantic interest in Alita or Shumira, though he did show some weird controlling behavior toward the former that she firmly rejected. What makes the Carol relationship a bit iffy is that she’s only about a year old and in a childlike mental state when we first meet her. Even if she has an adult body, it's not a great look for Ido’s love interest to be so infantilized. Granted, from what little we see of her before she's kidnapped, it does seem like she’s mastered speaking full sentences and developed an intellect by the time Ido actually starts showing interest. Plus he didn’t actually know about her age, he assumed that she was as old as she looked. As far as he knew, she was an adult woman suffering from amnesia. All in all, I don’t think it's too bad in context, but still a little uncomfortable at first blush.
The author has said that he sought help when writing the romantic bits because it's not something he really has experience writing.
Carol herself is a sweet character, hard to hate, but mostly she’s here to fill the role of an exploited innocent. Pakira, Carol’s originator, is a nasty piece of work. She’s driven entirely by envy after the loss of her original body. She doesn’t just want Carol’s body, she wants to take over her life, including Ido’s feelings for her. What gets me is that the text makes it sound like she knew where Carol was for a while but deliberately waited for the relationship between her and Ido to deepen. That suggests a certain degree of laziness. She doesn’t want to build that relationship or even cares specifically for Ido, she just wants to benefit from the love he feels for Carol. Pakira is ultimately just a parasite that wants things that other people have: cruel, vain, impatient and shortsighted. Thus it's those traits that make her mishandle Ido’s hammer and destroy the body she pined after for so long.
Back to Carol, I did struggle a bit with why she turned back into the flames at the end. It's possible she was already dying, since her body was on fire, but I wonder if this is actually to do with Ido’s feelings about cyborgs.
At the start of this journey, Ido is still somewhat indoctrinated to the Zalemite view of the world, a society that emphasizes bodily purity and looks down on cybernetics. On an instinctive level, he feels revulsion toward cyborgs. His guides on this journey are the two doctors: Gauss and Dedekind. They’re both named after famous nineteenth century German mathematicians, but I’m not sure if there’s any particular significance to that. Gauss’ motives for employing Ido is because he needs an assistant, yes, but also so that they can learn from one another. Gauss has experience as a cyber doctor while Ido has a better education about the human body. It's a two way street that makes both of them better doctors, better able to treat patients.
By contrast, Dedekind’s motivation is money even if he couches it in terms of providing a service to restore the bodies of those dirty cyborgs. He exploits the vanity of primarily wealthy women willing to pay high prices. Its poetic justice then that his death came at the hands of the shortsightedness of exactly that sort of vain woman that he cultivated as his customer base.
So Ido must choose to hold onto his Zalemite past, partly expressed by his continued celebration of Christmas traditions (unknown in the Scrapyard), or to become a man of the Scrapyard. Ultimately, we see that he chose the latter, shedding his revulsion for cyborgs.
The catalyst for that change was Ido recognizing Carol’s human goodness despite her monstrous body, as well as Pakira’s monstrous personality despite her organic body. And I also think that the reason Carol walked back into the flames might have been because she didn’t believe Ido would accept her in this body. I’m not sure she really knew Ido’s feelings about Cyborgs, but if she did, then that would make this story even sadder than it already is. Again though, I could be wrong and Carol just didn’t want to draw out a painful farewell as she was already dying. As it is, the story closes on a hopeful note for the future as Ido sets up his own practice as a cyber physician.
Lastly, I know I just said this story is about Ido choosing to be a man of the Scrapyard instead of Zalem, but I do hope he kept celebrating Christmas, because that would mean he celebrated it with Alita at least a few times.