Well Brittany if you really believe "who" is the subject you would be correct. "Who" is a pronoun which connects to the antecedent "girl." To check the difference, it is common practice for one to use "he/she" or "him/her" as acid test to decide between the words who or whom. Although "whom" would be objective especially in a prepositional phrase like "for whom," you could still say "for him/her." You certainly could not properly say "Her is that girl." So logically you could not ask "Whom is that girl?"
Another way to use "who" would be as a relative pronoun for a person. For example, the man
who lead the revolt was John Brown. For a person or thing you can use "that" too. The person
that led the revolt was John Brown. Otherwise, for things one would use "which." That is the toy with
which Sandy plays the most.
Hopefully this answers your question, Brittany. I also tutor Spanish...