What is the word "who"?
Is the word "who" an adverb? If not, what is it? If it is an adverb, what type of adverb is it?
Solution 1:
As Jon Purdy said, it can be an interrogative pronoun:
Who are you?
It can also be a relative pronoun:
I'm the one who was driving.
Solution 2:
It's an interrogative pronoun.