a frequent and formidable opponent
nemesis - rival
A ‘nemesis’ is a person or thing that is very difficult to defeat and frequently causes you major problems. The word can be used in a positive context to refer to a competitor with whom you are evenly matched, or in a negative one in relation to somebody or something that regularly gets the better of you.
I will be up against my old nemesis in the final. He and I have been competing against each other for many years.
The ongoing crisis is likely to be the President’s political nemesis in the upcoming elections.
Sarah’s nemesis when it comes to public speaking is her extreme anxiety.