I was thinking Phantom of the Opera. Either way, someone who needs some love.
Why can't it be "Who is behind the mask?" when you say "who", doesn't it already denote a person?
I could be wrong but I think the 있는사람은 is in the sentence to add emphasis on the person.
Because it is a whole phrase. The phrase is 가면 뒤에 있는 사람, which literally means 'mask behind existing person' (i.e. Person behind the mask).