[quote=@Zeroth]
Inertia's character sort of already does that, although rather than open flame the focus is on explosions. While the body-hardening would put a different spin on it, I'm afraid I'd really prefer if there were as little overlap as possible between the powers, sorry. I do like the idea of a body-hardening Quirk that uses different materials, though--maybe use some metallurgic knowledge of alloys or something to figure out another way to apply this kind of power? Is there such a thing as two metals that become magnetized or electrified when rubbed against each other, or something like that?
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I thiiiiiiink there is? Like, just doing a quick search on it, [url=http://classroom.synonym.com/magnetize-things-6377.html]iron, nickel, and cobalt[/url] seem to do it? If you strike it with a hammer or rub it around at least.

I really like the idea of a body-hardening Quirk with different materials too, but I'm honestly not interested in researching the science behind how that stuff works. I could just leave it as body-hardening, but it feels kinda boring to just leave it at that.