Pierre is a character only referred to in the first chapter of Buttercup's Baby (the supposed sequel to the novel), which is included in the 25th anniversary edition of the book.
He is in charge of the pirate ship Revenge during Westley's absence and next in line to become the Dread Pirate Roberts. By the end of the chapter, Westley and his companions leave the ship again, presumably leaving Pierre once again in charge; but no mention is made of an official transfer of the title of Dread Pirate Roberts.
Pierre is not mentioned in the film. Instead it is implied Inigo Montoya will take over the mantle of Roberts instead.