When you are asking about someone, you should use “Who is he?” or “Who is she?”
Is “Who is this?” at any time correct when asking about people?
Of course “What is this?” is correct when asking about things.
Depends on context, situation etc. But you might use "Who is this?" when receiving a phone call and you are not familiar with the voice of the caller, or you are looking at a picture of a person and you ask the question of someone who might know the person in the picture. I'm sure there are other examples. On the other hand, you might hear/use "Who is that?", referring to someone in the immediate area.