What kanji is this, and what's its meaning?
It isn't kanji.  The character you see here, and others, were invented for this particular TV series.  They're called ヨゴ文字 ("yogo moji"), and this character in particular is a substitute for the kana む ("mu").

See the following charts for details: kana and numbers 

As for what it means, given the context in your question, I would guess the む on his mask is 無 ("nothing").

(As an aside, these characters remind me of Tangut script.)