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Concordo com esta fonte :
Those heroes' normality is their greatest strength.
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Hawkeye and Black Widow in particular deserve to be Avengers because of their awesome skills. Hawkeye is a master bowman, Widow the finest spy in the world. In comparison to their allies they might just be human, but that's the point: they're essentially the audience surrogates in ideal form, the best a human being can be.
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To us normal folks watching and reading the comics and movies, these non-powered people are our gateway into this weird and intense world of gods and heroes. They're humans at the peak of their potential, standing as equals with the superpowered. And that's something to look up to: the fact that at our best, we can stand toe-to-toe with the likes of Thor or Captain America.
And way fewer people ever bring up these questions when it comes to the ultimate manifestation of the 'badass normal': Batman. Part of Batman's whole mystique is that he is so finely honed, so well trained (and so well endowed with wonderful gadgets and toys) is that he can stand tall alongside aliens and people who are basically walking gods.
If Batman is so talented, so amazing that he can take down foes powered and unpowered alike, it magnifies the abilities of the likes of Wonder Woman or Superman to the awe-inspiring levels that we should be seeing them at. Without an extraordinary human alongside them, superheroes risk becoming sort of ho-hum and aloof, because they become the new normal. Standing alongside a regular human being grounds them in such a way that their superpowers stand out more.