Is who a common noun?

Answer 1

No, the word 'who' is not a noun. The word 'who' is a pronoun.
The pronoun 'who' is an interrogative pronoun and a relative pronoun.

An interrogative pronoun introduces a question. The antecedent to the pronoun 'who' is normally the answer to the question.
The pronoun 'who' is the subjective form.

A relative pronoun introduces a relative clause, a group of words with a subject and a verb but is not a complete thought, is not a complete sentence. A relative clause gives information about its antecedent.

Examples:
Who gave you this assignment? It was my history teacher. (interrogative pronoun)
Ask the teacher who assigned it your question. (relative pronoun)

Note: The objective form is 'whom' which normally functions as the object of a preposition.
Examples:
To whom should I give my completed application? (interrogative pronoun)
The one to whom you give your application is the manager. (relative pronoun)