The "scary-looking man" is a Greek native.
He is a middle-aged Caucasian. He wears a tan trenchcoat over a pale blue shirt, black pants, and brown shoes.
He seems unhappy with the gang at first because they think he is cursing them, however, he is only trying to be nice and return Daphne's purse.
What's New, Scooby-Doo?
The gang met him when they came to Greece. His scary expression made him a suspect in their eyes. Because of him speaking in Greek to them, Mystery Inc. thought he was trying to curse them and try to get away from him. The Greek man kept following them all around Greece until Lysander Demas explained that he only wanted to return Daphne Blake's purse ("Thélo na sas dósei píso ti chaméni tsánta sas!").