Is this Dennis?
I know that our Dennis the Menace has a blonde counterpart in North America. I seem to recall that he was more mischievous than menacing.
Spot-on ! British Dennis, a right scallywag, though of a non-vicious kind; the American one an altogether milder sort, often more thoughtless than actively mischievous. As you say, black-haired and fair-haired, respectively.
Most weirdly, the two comic strips were launched in a British comic, and a US one : within days of each other in March 1951 -- apparently no collusion whatever, just a remarkable coincidence.
Please whistle up your dog -- Gnasher or Ruff, according to choice -- and set something with which to baffle us !