Who is Marcia Lynn Dorsett from Sybil and what is their importance?

Marcia Lynn Dorsett appears in Dr. Wilbur's office shortly after Mary, but she does not appear alone. Marcia and Vanessa go everywhere together. They inhabit Sybil's body at the same time. Dr. Wilbur is admittedly shocked by this development, but she is even more surprised that she can easily distinguish between the two girls. Marcia talks with an alto English accent, loves to travel, and has a great sense of humor. Marcia is a writer for the Coronet.