Russian mathematician who solved the Poincaré conjecture, then declined both the Fields Medal and a $1M Millennium Prize. Lives reclusively in St Petersburg.
From Wikipedia
Grigori Yakovlevich Perelman is a Russian mathematician and geometer who is known for his contributions to the fields of geometric analysis, Riemannian geometry, and geometric topology. In 2005, Perelman resigned from his research post in Steklov Institute of Mathematics and in 2006 stated that he had quit professional mathematics, owing to feeling disappointed over the ethical standards in the field. He lives in seclusion in Saint Petersburg and has declined requests for interviews since 2006.