Monkey D. Luffy is a boy from Windmill town. At the time, a group of pirates (Red Hair Pirates) that is lead by Red Hair Shanks were staying there for a while. Coincidentally, Luffy has eat a demon fruit that was brought by the pirates. The rumors that said who eat devil fruit would get unique ability but cannot swim is indeed true. Thanks to the Gomu Gomu devil fruit, Luffy got a rubber ability that enable him to stretch himself like rubber, but he cannot swim anymore. In One Piece, you would see Luffy creativity in using his rubber ability. Luffy fighting technique is something that I really look into.
Luffy decided to become a pirate because he once was saved from death by Shanks. As the result of saving Luffy, Shanks lose his left arm. That happens when Luffy is still small. So that Luffy didn’t forget his promise, Shanks lend his straw hat to Luffy and ask him to return it once Luffy has become a great pirate. Starting onwards, Shanks doesn’t wear any straw hat. And starting from that, Luffy keeps in mind that he wants to be a pirate king.
A few years after the incident, Luffy sail to the sea in order to become a real pirate and recruiting crew members. He then perform a crew members from people that he find through his journey. Most of them have their own reasoning in joining Straw Hat Pirates. With that Luffy has become the leader of Straw Hat Pirates.
Although Luffy is the captain, Luffy does not make most decision. Based on the manga that I read, it looks like most of the decisions is made by Nami. This cannot be helped since Luffy is someone who is not matured enough. He is a happy go lucky and simple minded person. However, there is also special case where you can see Luffy going berserk and can be depend on. Especially when his ‘nakama’ (friend) need help. For Luffy, his nakama is more important than his life. In One Piece you can how much Luffy struggled in order to save his nakama especially during Nami case at Arlong Park, and Robin issue with CP9 at Enies Lobby.
