Who is Margaret Porcher from The Woman in White and what is their importance?

Margaret Porcher is described as huge, fat, and impenetrably stupid. She is employed as a maid at Blackwater Park. She is later assigned to guard Laura from leaving her room to see Marian at Blackwater Park during Marian's illness. Sir Percival keeps her because she is too stupid to object to the orders he gives. She has no ethical standard but merely follows orders.