A "bünzli" is a person who follows rules at all times and wants others to follow them, too. It's usually also associated with people of higher socioeconomic status or older people. It has a partial overlap in meaning with the English slang term "Karen" in that a "bünzli" will also talk to a manager (or some kind of higher official) if something isn't done their way.
"Mi nochber isch sone bünzli. Är het gest dr polizei aglüte weil mr am 22:30 ufem balkon musik glost hän."
"My neighbor is such a bünzli. He called the police yesterday because we were listening to music on our balcony at 10:30pm."