Who is Ratchett (Casetti) from Murder on the Orient Express and what is their importance?

Ratchett is the murder victim aboard the train, as well as the source of the mystery that anchors the narrative. Ratchett is American. Poirot describes him as somewhat animalistic, and he investigates his death, Poirot discivers that Ratchett is in fact a notorious kidnapper named Cassetti.