Luffy has a worldwide reputation for causing trouble, challenging the Shichibukai, the Marines, and even a Yonko, committing felonies against, as well as actions which were deemed threatening by, the World Government. He has gained a reputation for being "reckless" and, in some cases, "insane", after causing an incident in Enies Lobby, entering and escaping from Impel Down, and joining the Battle of Marineford.
He is, therefore, famous for being the only pirate to not only break into the three most important government facilities, but also escape from all of them alive. Including the fact that he punched a Tenryubito with full knowledge of the consequences, as well as his heritage, has caused Luffy to be labeled as a "Dangerous Future Element", earning the wrath of Fleet AdmiralSakazuki, making the Marines among his greatest enemies.
These acts, amongst other things, have given him his current bounty of 400,000,000, one of the highest past or present bounties in the series. Having had a bounty of 300,000,000 prior to his arrival at the Sabaody Archipelago, Luffy was one of eleven rookie pirates who had been referred to as "The Eleven Supernovas", the eleven pirates with bounties over 100,000,000 to have reached the Red Line.
Luffy is renowned for his trademark straw hat (from which he gets his nickname "Straw Hat Luffy"), which was lent to him when he was young by the legendary pirate captain, "Red-Haired" Shanks, who in turn received it from Gol D. Roger. He wears short trousers along with sandals and a sleeveless red vest. Luffy also has a scar with two stitches underneath his left eye (which he earned by stabbing a knife under his eye to show Shanks that he was tough) and short black hair. He was severely wounded by Akainu in the Whitebeard War, leaving a large X-shaped scar on his chest. Appearing to be scrawny underneath his shirt, he has a surprisingly well-built physique. In the pre-timeskip, he was short since he appeared quite dwarfed by most individuals in the series.
Before the Timeskip
Similar to most of his male crewmates, Luffy has the same outfit throughout most of the series, as opposed to the rest of the crew (especially Nami and Nico Robin), who change frequently. Luffy may wear a different outfit to suit the climate of an island (like a hooded parka on Drum Island or desert robes in Alabasta), but will switch back to his trademark vest and shorts after the adventures on that island are over. Despite this, he will wear his usual outfit in different colors and has been known to sport other miscellaneous items on his person from time to time.
During the Drum Island Arc, he wore two parkas, the first being a dark red parka with fur on the cuffs, hood, and hem, and then a checkered, hooded parka with the colors white, orange and bright yellow, which Nami wore in the beginning.
During the Alabasta Arc, he wore desert robes consisting of a dark red robe with three white rings decorated along both of the sleeves and a light blue wrap over his head.
During the Skypiea Arc, he wore a black wristband around his left wrist with a white bangle over it.
During the Long Ring Long Land Arc, Luffy wore an afro on his head, dark red shorts, and red bandages on his feet, as well as getting an image of a blue flaming skull and crossbones painted on his chest when he was fighting Foxy.
During the Enies Lobby Arc, he wore black shorts because they had pockets in which he could carry meat.
During the Post-Enies Lobby Arc, Luffy wore a white T-shirt with the Galley-La Company emblem on it in red, along with red liners around the top the shoulders and collar, and navy blue shorts.
During the Thriller Bark Arc, he wore an orange vest and red shorts. After the battle against Moria in Thriller Bark, Luffy wore a dark decorated, jeweled armband around his left bicep. This armband was later revealed to be a map that leads to Captain John's lost treasure, which Buggy sought for some time - leading to Luffy handing the armband in for Buggy in exchange for his services at helping him navigate through Impel Down.
During the Sabaody Archipelago Arc, he wore a blue vest and red shorts in the manga, in the anime he wore a blue vest and dark blue shorts.
During the Impel Down Arc, Luffy wore his outfit from his stay on Amazon Lily until after his fight with Magellan, moment in which he replaced his light blue vest with a bright yellow vest given to him by Inazuma.
No matter where he is, Luffy wears his Straw Hat through absolutely everything, though at the start, the hat was prone to getting knocked off his head easily, forcing Luffy to either put it aside or have someone hold onto it during fights. From the Alabasta Arc onwards, he has gotten a string strap for it, which was sewed on by Nami, allowing the hat to hang from his neck during fights.
During his return to Marineford to give a silent prayer, Luffy was briefly shown with four characters written on his right arm spelling 3D2Y, with the 3D crossed out to symbolize that the Straw Hat Pirates should meet up in 2 years (2Y) rather than the 3 days (3D) as planned after fleeing from Bartholomew Kuma.
After the Timeskip
After those two years, a few things have changed. Luffy wears an open, long-sleeved red cardigan with four buttons (which shows the X-shaped scar covering most of his chest he received by Admiral Akainu), with a yellow sash tied around his waist, somewhat reminiscent of Gol D. Roger's outfit. He has grown slightly taller, and is shown to be significantly more muscular due to his training. This can be seen in instances where his neck is slightly thicker, more pronounced deltoids, and his chest also having more definition.