Good canon grounding to work from: book-Melissa is described as looking delicate — pale, small, "like one of those solemn elves in a Tolkien book" — but with real strength underneath once you look closer. Her parents (in canon) both go Controller specifically to shield her, which means in your AU the more interesting move is that she notices something changed about them around the same window and never got a real explanation. That's a strong, plausible seed for a kid going quietly feral on her own timeline. What she plausibly figures out, and what she doesn't: she's not going to land on "aliens" or "parasites." A smart, isolated 15-year-old in 1998 lands on something closer to "my parents were replaced" or "my parents are compromised by something — cult, blackmail, mind control, I don't know which." That ambiguity is useful — it's why she reads as a wildcard rather than someone who's solved the puzzle. She's operating on pattern-recognition (personality shift, secrecy, specific behavioral tells) without a framework for what she's seeing. What a resourceful, unsupervised kid could actually build in a year, 1998 Santa Barbara:

Research tradecraft that actually existed then — library microfiche, newspaper morgue searches for missing-persons or unexplained-death patterns, becoming a regular at the reference desk asking oddly specific questions. No internet trail to worry about, which fits her paranoia. Physical competence — if she's got access to some money (assistant principal's household isn't poor, even if emotionally absent), a self-defense or martial arts class is completely plausible and unremarkable to enroll in. Girl Scouts / outdoor-ed exposure (common for her generation) gives her a legitimate excuse to have camping gear, a knife, know basic first aid and navigation. Surveillance habits — keeping a notebook of her parents' schedule anomalies, learning to read a room before entering it, memorizing exits. This is more character trait than acquired skill and it's the most narratively useful piece. Financial squirreling — babysitting/odd-job cash kept separate from any account a parent could see, which tracks with prepper logic and doesn't require any specifics I'd need to hand-wave. I'll flag one thing rather than write toward it: I'd keep the sawn-off's backstory vague on the page (found it, inherited it, someone she did work for gave it to her) rather than dramatizing how a 15-year-old sources and modifies a firearm — that's less a story-craft note and more that the specifics would be real-world actionable in a way that's outside what I'll spell out. The fact of the weapon and what it signals about her is the part worth writing; the acquisition mechanics aren't necessary to show.

Personality — where I'd differentiate her from Sophia: you're right that the register rhymes, but I'd push for this to read as a different phase of the same trajectory rather than a rerun. Sophia in Mixed Feelings is post-collapse — she's already been ground down by sustained loss, and the flatness is exhaustion after the fact. Melissa hasn't broken yet. She's pre-collapse: not flat from exhaustion but flat from discipline — a kid who has decided that feeling things out loud is a liability and has gotten good at suppressing the tell. Concretely that might mean:

She still has functioning routines (school, a thin social performance) that Sophia's already abandoned — she's maintaining cover, which takes active effort and gives you scenes Sophia's story doesn't have room for. Her hypervigilance can have moments of leakage — a flinch, a too-fast hand toward the backpack — that she visibly claws back under control, rather than Sophia's more total numbness. Keep her affect controlled rather than absent. Sophia's gone past caring what people think of her stillness; Melissa should still be performing normal, badly, at a cost that shows.

That gives you a throughline (isolated girl, quiet competence, violence-adjacent readiness) without making her a retread — she's the version of that arc caught earlier, still paying the price of the mask rather than having discarded it.