A) If the pronoun is the subject of a sentence, use he. If the pronoun is the object of a sentence, use him. Your example should be: Who is he. An example where you would use both: I’ve seen him – who is he? B) If someone called you on the telephone and you do not know them you might ask: Who are you? or Who is this? For some reason I can’t explain, the question “who is it?” is not idiomatic …
