The main designer is Andreas Rumpf, but investigating the git commits of the new Nim reveals more people being involved. [1] Whether Andreas is a genius, I have no idea, but he has been doing compiler and language development for over 20 years [2] so he's probably extremely knowledgeable regardless.

[1] https://github.com/nim-lang/nimony/commits/master/

[2] https://en.wikipedia.org/wiki/Nim_(programming_language)