Luffy has a worldwide reputation for causing trouble, challenging the Shichibukai, the Marines, and even a Yonko, as well as committing felonies and actions which were deemed threatening by the World Government. What's more, in the majority of these confrontations, he emerges as the victor. He has gained a reputation for being "reckless" and, in some cases, "insane", after causing an incident in Enies Lobby, assaulting a World Noble at Sabaody Archipelago, entering and escaping from Impel Down, and joining the Battle of Marineford.
He is, therefore, infamous for being the only known pirate to not only deliberately attack the three most important government facilities, but also escape from all of them alive. The additional 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 Admiral Sakazuki and making the Marines among his greatest enemies.
These acts, amongst other things, have given him his current bounty of Beli400,000,000, one of the highest past or present bounties in the series. Having had a bounty of Beli300,000,000 prior to his arrival at the Sabaody Archipelago, Luffy is one of eleven rookie pirates who has been referred to as the "Eleven Supernovas", pirates with bounties over Beli100,000,000 who have reached the Red Line. He is also one of the pirates widely regarded as The Worst Generation.
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, blue trousers with cuffs, 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, messy black hair. He was severely wounded by Akainu in the Battle of Marineford, leaving a large X-shaped scar on his chest. Appearing to be scrawny underneath his shirt, he has a surprisingly well-built physique. He is quite short since he appears 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 red vest and blue 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.
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 then-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.
During the Punk Hazard Arc, Luffy wore a long winter coat with red and white horizontal strips, along with yellow ear muffs, which he stole from one of Brownbeard's Centaur Patrol Unit.
During the Dressrosa Arc Luffy wore an open navy short sleeved shirt with a sunflower pattern, along with his standard blue shorts and yellow sash. He also wore a fake beard. Upon entering the Corrida Colosseum he wore viking style armor, complete with a sword and shield, until it was stripped from him due to violation of the weight restriction. Upon revision to the weight restriction he added a cape and wore the same viking style helmet he wore previously. Along with his registration number '0556' on the back of his shirt, obscured by his cape.