Nai Nai. Melty's abilities are more physiology than Nukas' character. While Nukas secrets slime from his body and morphs his body into slime, Melty is just sentient slime, period. She doesn't have internal organs or any of the like, so the need for oxygen is moot. What she relies on is hydration, which is another issue altogether when paired against air manipulation.
The second point is true though, she wouldn't exactly do well in a Vacuum. However, she wouldn't rise to a boil instantly. She does have the ability to modify the viscosity of her slime, so she wouldn't be going poof in an instant. I'd probably have to play to increase the intermolecular bonds between her slime to counteract the changing air pressure to keep her liquid state though. Or instead, I'd create a membrane akin to the skin through means I haven't quite thought out yet lmao. That way even with her internals at a boil, she'd still be cohesive enough to function.
I'd probably need to have her adopt the characteristics of an amorphous solid, though let's talk about that once I've panned it out in my head lol.
At the same time though, Sky doesn't function well in a vacuum. CoDA's Sky, and the other iterations of him that I've seen aren't 100% air since they actually come with a massive weak point called his core. By my understanding, the creation of a vacuum would essentially leave his core exposed for someone to come up and start railing on.
I do indeed figure that localized vacuums might be possible, but not in very close proximity to his body.
Following that line of thought, since air will be present around his immediate core 24/7, sound manipulation is a clear shot as his weak point. Of course, this doesn't account for the creation of a vacuous shield or anything.
Edit:
Also, Straight up? Face to Face? Are you mad? Having Melty do that against anyone is tantamount to giving up.
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