"a law unto themselves"

If somebody is a law unto themselves, they do things their own way and follow their own ideas about how to live instead of following what others do.

Our boy Billy is a law unto himself. Of course he sometimes makes mistakes and gets into trouble, but we're proud of him because he doesn't just follow the herd.

Our boss does things a bit differently to most other managers, so it's no surprise that some people say he's a law unto himself.