So far there are only 4 people we can garuntee are NOT the Red Devil.
Zayday - She was kidnapped by Boone and escaped, she is seen taking the mask of off the rd in the last episode, she is a totally different ethnicity (I mean...they could be adoptive siblings? But then why would Boone kidnap her?)
Dean Munsch - She is WAY too old to be Boone's sibling, she was attacked by all 3 killers (Boone, Gigi + twin sister)
Chanel Oberlin - She was attacked by the last RD in the Black Friday episode
Denise Hemphill - Was with Chanel in the mall when she was attacked by the RD, too old to be a sibling, is a guest star....could she be the RD if she isn't a main cast member?