It is a pronoun and is technically the subject. Generally, the object form of the pronoun is "whom", but the word "whom" is beginning to become a bit of a relic in modern English, so using this as a steadfast rule to determine between the two wont always work. Hope this helped :)

Who is a pronoun, and it is the subject. As far as I know, it is always the subject of a clause. It can be confused with whom, which means the same thing except it is used as an object. Examples: Who shot the sheriff? I shot the sheriff. Whom did you see at the mall? I saw a man dressed as Santa Clause at the mall. I hope this helps!

"Subjects" and "pronouns" are not mutually exclusive categories. The subject designates the actor in a sentence, and can either be a noun or a pronoun. A pronoun is a part of speech, and does not a priori determine what part of a sentence it occupies (it could be a subject or an object). So the answer is that, in "who is that," the word "who" is *both* the subject of the sentence and an interrogatory pronoun.