Who is Rachel from The Fifth Sacred Thing and what is their importance?

Rachel is Madrone's mother. She was killed in Guatemala, when Madrone was a child in an act that Madrone suppresses. Although Madrone communes with other spirits of the dead, she notes several times that she has never dreamed of her mother. However, Madrone realized that the visions she has had of La Serpiente are her mother. Rachel appears to Madrone when she is almost drowning in the ocean and later when she is on Isis's boat, after the rescue of Katy.