In many situation such as:
A) If friend tells you about someone else that it's not clear to you who is the person that your friend talks about.
B) Someone calls you and you do not know who is this person.
In some languages, the question will be translated into "Who is it?" using the pronoun it about a person(!). Is it the same in English, or should we say "who is he?"?
If you've got totally no idea who that person is, not even the gender, then you could say 'who was it?'. And also, for 'who was it', you can use it when your friend answered a phone call and then he hangs up the phone. Then, you ask him 'who was it?'
For situation A, since your friend is telling you about that person, you might ask 'who is he or she'
But, I don't think situation B makes any sense as you may just ask that person 'who are you?'.
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 when asking someone directly on behalf of yourself. It only seems to be used if asking for someone to identify themselves through a closed door, and also if asking on the telephone for somebody else, eg:
Caller: Can I speak to John, please?
Respondent: I will get him. Who is it?
