Is it rude to ask someone "Who are you?" at the first time of meeting?
Yes, in general it is impolite to say "who are you" when you are first meeting someone. It is much more polite to say "what's your name". When the boy asks "who are you" in the video here, he says it in a way that makes it seem like he doesn't want to see the man and is annoyed that he has come to visit. If you ask someone who they are when first meeting them, it can sound like you are annoyed to be meeting them and you don't like them.
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