How would you describe someone who doesn't worry about what other people think? Someone who has the confidence to think for themselves. It could be a quality or adjective. One word preferred
Maverick was popular in the last U.S. presidential election cycle, though the candidates to whom it was applied would probably not be called free spirited or nonconformist, which have whimsical and countercultural connotations respectively.
If individual always acts without external input, I'd call him or her a lone wolf.
If s/he simply can't be bothered with external input, nonchalant.
If s/he wants to strike down convention actively, iconoclastic.