So, Lindsay Ellis has a whole video examining the character of "The Persian" from The Phantom of the Opera, and why he is (almost) always cut out from any film and stage adaptations.
This was something I also used to wonder about as a youngster.  I read the novel The Phantom of the Opera when I was in 7th grade, and I also noticed that The Persian was very prominent in the novel, but never in any of the adaptations.
Of course, my knowledge of Phantom of the Opera adaptions was not as extensive as Lindsay's.  As a kid, I knew of the 1925 Lon Chaney version by reputation (it was mentioned in every book I ever read on classic horror films), but was never able to locate a copy of it.  I saw the 1943 Claude Rains version on cable on night.  And I knew about the Andrew Lloyd Weber musical, even though I never saw it in person.
Lindsay, on the other hand, was full into Phantom fandom.  Which makes her videos all the more interesting.

The Most Whitewashed Character In Literary History