Luffy is the captain of the Straw Hat Pirates and, considering them his best friends, values them over all else. Even though they are all usually annoyed by his immaturity and simple-mindedness, they all respect him for his strength and his qualities as a leader.
At first glance, Luffy does not appear to be very intelligent, as he often sees things in childish manner and can easily be amazed by the simplest things. However, because he views the world in such a straightforward and simple manner, he is frequently the only person who can see what should be done. He likes to poke fun at other's imperfections and character traits, even though he is usually no better than those he mocks.
Luffy seems to have an insatiable appetite, a characteristic that is common to the Japanese archetype of the (at times simple-minded) young male hero/adventurer with a heart of gold; perhaps hunger in Luffy's case has to do with having an elastic stomach.
Although Luffy is fairly lighthearted, he has an unstoppable sense of determination, optimism, and courage, and will do anything to stand up for his friends and comrades. Ever since consuming the devil fruit he has not worried about his inability to swim, much like his brother. Many of these traits are common among Ds. The only thing which seems to frighten Luffy is his grandfather; the mere mention of him intimidates Luffy to such an extent that he does not fight back.
Luffy never kills any of the villains, no matter how cold-hearted they may be; instead, he frequently sends them flying, knocks them out, or leaves them beaten into submission (consequently, it is not uncommon for Luffy's former opponents to come after him for revenge, such as Buggy the Clown and Alvida). Oda explains that it is not a question of morality so much as a matter of punishing the villains for their crimes - he feels that killing the villains lets them off too lightly, whereas allowing them to live and see their dreams ruined is a far more fitting punishment.
Luffy's dream is to find the One Piece and become Pirate King. He knows that to achieve his goal, he will have to defeat many strong opponents, including the World Government and his childhood hero Shanks.
