What purpose is served by introducing Mrs. Rogers as a frightened "ghost" of a woman?

The purpose the author has of introducing Mrs. Roger as a
frightened ghost of a woman is to add a sinister, foreboding feeling to the
atmosphere. Vera had just walked into a delightful bedroom with a big window and was
very pleased. Mrs. Rogers escorted her there. But when Vera looked at Mrs.
Rogers closely, she thinks Mrs. Rogers  is very pale and "bloodless".  Although she was
quite respectable and dressed as a maid, she had 

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"Queer light eyes that shifted the whole time
from place to place" (pg
31)

Vera thinks she looks
frightened of something and that idea gave Vera a physical reaction, it made a shiver
run down Vera's back. She says that

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 "She looked like a woman who walked in mortal
fear..." (pg 31)

The word
mortal means subject to death. (See link)   It
gives the reader a feeling of dread of some kind of evil force or forces on the island
that are going to cause someone's death.  When Mrs. Rogers  turns to leave the
room,

"Her
feet moved noiselessly over the ground. She drifted from room to room like a shadow" (pg
32) 

That is another
reference to a ghost.

The pages of the book may differ
depending on which edition you are reading, but the words should be in the general
vicinity of the pages listed.