WHO is Rachel of the Gothic romance mystery, My Cousin Rachel?

First, meet Sam Claflin (The Hunger Games) as 24-year-old Philip Ashley, an orphaned aristocrat in mid-19th century England raised from young by his adored wealthy landowner cousin Ambrose (also Claflin in brief flashbacks). Ambrose, while in Italy's Florence to nurse his failing health, meets and marries Rachel, another distant cousin. His love-struck letters home soon turn paranoid, claiming his bride is wicked and predatory and begging Philip to come rescue him.

Too late. Ambrose is suddenly dead, murdered, Philip believes, by Rachel.