He's a hacker who perhaps most notably claimed to have hacked into Hillary Clinton's private email server.

https://en.wikipedia.org/wiki/Guccifer

Russian source just for fun: https://www.rt.com/usa/347681-guccifer-clinton-dnc-defense/

Hacker 'Guccifer 2.0' publishes DNC campaign docs with strategies for defending Clinton

The handle is a reference to https://en.wikipedia.org/wiki/Guccifer