The word ‘who’ is primarily used as a pronoun. It is an interrogative pronoun when used to ask questions about individuals, and it can also function as a relative pronoun in clauses to refer back to a noun.
For instance, in the question, ‘Who is coming to the party?’, ‘who’ is serving as an interrogative pronoun, asking for the identity of a person. Alternatively, in the sentence ‘The man who called you is waiting outside,’ ‘who’ is a relative pronoun that introduces a clause providing more information about ‘the man.’
Therefore, ‘who’ can flexibly serve different roles in sentences based on how it’s used, but it is classified as a pronoun overall.