I was thinking Phantom of the Opera. Either way, someone who needs some love.

Yes, but the Korean sentence specifically says 사람.

i already reported that

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).

That would be a veil.

Your missing a "the" between "Who is the person". Duo is pretty strict with articles.