Who is Mr. Prewitt from Brighton Rock and what is their importance?

Mr. Prewitt is an aging, dyspeptic, slick lawyer, the legal aid to Pinkie's gang. He arrives midway through the action to arrange the wedding of Pinkie and Rose. While at Frank's boarding house, Prewitt becomes the unwitting witness to Spicer's murder. He is forced by Pinkie to provide testimony that Spicer's death was an accident. Later, Pinkie becomes convinces that Prewitt is losing control of his sanity and pays his way to a holiday in France. Pinkie has one of his men follow Prewitt to the boat to make sure he board.