Monkey D. Dragon is one of the most influential men in the world of One Piece, given that he is the leader of the Revolutionary Army and, he's also extremely important to the plot for that reason, and because he's Luffy's very own father. Dragon was first introduced in the series during the Loguetown arc, however, his proper introduction came after the events of Enies Lobby, where Garp mentioned to Luffy that Dragon was in fact his father and the leader of the Revolutionary Army.