Luigi

Luigi is very similar to his older brother in appearance, although Luigi is slimmer and taller. His face is notably thinner and chinless. Luigi has a smoothed mustache and light blue eyes. He wears unfaded denim jean overalls, brown work shoes, and a green long-sleeve shirt. Luigi's Hat is similar to Mario's Hat, being green instead of red and having an "L" replace Mario's signature "M." Artwork for the Mario & Luigi series has consistently depicted Luigi as wearing red and white striped socks; this trait is not present in any other games.
Mario and Luigi's color preferences are reflected throughout the land in the form of various enemies, such as Koopa Troopas and Cheep Cheeps, who also come in red and green varieties. These foes even reflect the brothers' personalities, with red ones being more aggressive and green ones acting cautious. In fact, Mario and Luigi are not the only known red and green brothers, with similarly color-coded siblings including Cork and Cask, Red and Green, the Armored Harriers, and Gigi and Merri. When tattling the younger of the Armored Harriers, Goombella theorizes that it may be some sort of rule that younger brothers must wear green, offering a possible explanation for Luigi's trademark green shirt and hat.
Bio from The Adventures of Super Mario Bros. 3 Writers Bible
Mario couldn't have a better teammate than his younger in age, but bigger in size, little brother Luigi, even though they approach their adventures with very different attitudes. Luigi always wishes Mario would take more time to weigh the risks before he leaps, swims, or flies into the unknown. But no matter how much he questions Mario's plans, Luigi always loyally follows him into the wildest dangers any Koopa could conceive. Luigi is your basic soft touch. A defender of downtrodden Mushrooms and Koopa-nabbed maidens. Naive, trusting and innocent, Luigi is not a bumbling fool. It's just that he's so nice and straightforward, which allows him to be sucked into a Koopa scheme, a Toad prank, or a Mario madcap plan.
However, Luigi has not always chosen to wear his usual green and blue clothes. Mario Bros. originally dressed Luigi in a black shirt and green overalls, although artwork showed his shirt to be red. Wrecking Crew egregiously gave him a magenta pair of overalls and helmet, seemingly leaving him shirtless. Even Super Mario Bros. gave its Luigi a green shirt and white overalls (although Super Mario Bros. Deluxe replaced this with a less aberrant brown shirt and green overalls, leaving his original colors to Fire Luigi, who usually has an inverse scheme), despite the fact that his usual palette and appearance had emerged in artwork.
Luigi is rather infamous for being a palette swap of Mario in his earlier adventures. This applies to Mario Bros., Wrecking Crew, Super Mario Bros., Super Mario Bros. 3, Super Mario World, Super Mario World 2: Yoshi's Island, and even the rare solo quest that is Mario is Missing!. Luigi's first unique sprites came in Super Mario Bros. 2, in which his artwork appearance conveniently coincided with the tall Mama's sprite. Nintendo of Japan would not adapt Luigi's artwork differences to his in-game sprite until Super Mario Kart, although Luigi's various models have been unique almost ever since, even in remakes of games where he was originally a palette swap (such as Super Mario All-Stars, Super Mario All-Stars + Super Mario World, Super Mario Advance 2, and Super Mario Advance 4).
Both Mario and Luigi have been described as "cutesy," to the extent that Shigeru Miyamoto considered redesigning them to "become a bit more grown-up" in the GameCube era, for instance by removing their trademark V sign. However, aside from growing more realistic as graphics have advanced, Luigi and his brother have changed little over the years, and continue to flash the V sign regularly.
Luigi's appearance does, however, tend to lend itself to heavy stylization. The Luigi's Mansion graphical team took it upon themselves to animate every part of Luigi to convey emotion, resulting in such exaggerations to the plumber's body as head-expanding screams during boss cutscenes and complete flattenings by fake doors. The Super Smash Bros. series applies Luigi's cowardice to his actions, making most of his attacks look reckless and unwilling and giving him awkward movements, securing his official title as the series' "dreamy, comical poster boy."
Luigi is portrayed as timid and a bit of a coward. He also worries and is considerably self-conscious. However, he overcomes his fears and acts like a hero when necessary. On the New Super Mario Bros. Wii website, it describes Luigi as "Mario's fearless brother". In keeping with his personality-based Power Flower, a sign in Big Boo's Haunt in Super Mario 64 DS describes Luigi as "wispy."
Arguably, Luigi's most apparent trait is his seeming cowardice. This was not seen until Luigi's Mansion (aside from a few hints in Paper Mario), but it has carried over to almost all of Luigi's subsequent appearances. Although Luigi's phasmophobia is his most overbearing fear, he usually reconsiders at the mere thought of any monsters or trials, especially in the Mario & Luigi series. However, Luigi can be quite brave, and has saved the Mushroom Kingdom alongside his brother multiple times. Super Paper Mario especially glimpses into Luigi's more courageous side, showing him boldly rushing into dangerous situations and rarely even implicating his cowardliness.
Another strong element of Luigi's personality is his apparent complex over being overlooked. Although his Superstar status has earned him many fans, including an entire fan club led by Toadia and multiple supporting Toads (one of which claims to be his biggest fan in Super Mario 64 DS), even Bowser sometimes has trouble remembering his name (as seen in Mario & Luigi: Superstar Saga), and he appears to be almost unknown in the Beanbean Kingdom. Although Luigi's biggest reactions to his frequent overlookings in the Mario & Luigi series are a few "..."'s and facefaults, his inattention was played up for comic relief in Paper Mario, with the snarky man in green complaining about being left behind and perpetually training for that elusive next adventure. Luigi's bio in Mario Party 5 states that he has been gaining popularity in recent years.
Luigi's state of being in the shadow of his brother shows strongly in Super Smash Bros. Brawl. An example would be the way Luigi is almost never happy, how his commentary is very monotoned, and how one of his taunts is a bashful kick of the ground. Also, his Final Smash, Negative Zone, seemed to have developed because of living in his brother's shadow forever. Additionally, whenever the crowd cheers for Luigi, not too many people are heard cheering for him and when the announcer says Luigi's name, he sounds a little surprised. Despite this, however, Masahiro Sakurai did not intend to play up Luigi's overshadowed status, instead aiming to make him "everyone's favorite man in green."
When Luigi finally went on a quest of his own in Paper Mario: The Thousand-Year Door, he's presented as be more arrogant and somewhat of a liar, as he changes the stories of his adventure to make himself look better; his partners claim his stories are false. The novelizations of his escapades describe them with even more flowery exaggerations, stating for instance that he played an earth spirit in the Jazzafrazz Town Drama Slam (whereas he actually played grass on the side of the road, according to Hayzee) and that he awoke Cranberry with "a soft call" (rather than a sneeze).
In Mario & Luigi: Partners in Time, the Star Gate (which supposedly knows all) passes Luigi as "unworthy" to enter. This most likely has something to do with Luigi's "dark secret from his past," which it also mentions; Luigi did something either embarrassing or foolish which he has kept a secret ever since. However, in a interview with a member of staff, he stated Luigi had no dark secret, and it was actually a reference to the fact Luigi constantly lives in the shadow of his brother. He also added: "Why make Luigi into a bad guy? We have Waluigi for that instead!" Even the game itself makes a straightforward point that Luigi has a large heart, as proven when he hits the Aurora Block and it grows to a gigantic size, flattening him.
Luigi is also portrayed as somewhat refined, leading the "Luigi Gentlemen" in Mario Superstar Baseball. Luigi's Mansion shows him to be a bit of a neat freak, constantly criticizing the dustiness of his mansion and even mentioning at one point that he handwashes his cap as opposed to using a washing machine as his brother does. He also seems to have an eye for art, but doesn't appreciate many of King Boo's decorations. Like Toadette, Luigi can be sensitive when someone insults him, especially if the jab in question regards his mustache (as proven prior to his showdown with Dimentio).
Whereas Mario rarely speaks aside from numerous yells and grunts (a point is made out of this in Super Mario RPG: Legend of the Seven Stars with Toad and The Thousand-Year Door with Doopliss), Luigi is proven to be quite the conversationalist in the Paper Mario series. In Super Paper Mario, Luigi is almost Mario's "mouth." He does all the speaking for Mario in the beginning of the game (a role later taken by Tippi). Once he joins Mario's party, however, he only talks during scenes where the four heroes are shown together. Curiously, the inverse seems to be the case in the rare instance of a Luigi-only quest; both brothers are capable of captioned speech in Luigi's Mansion, but Luigi is limited to ellipses and question marks when interacting with another character. In the Mario & Luigi series, Luigi, as well as Mario, talks in gibberish that the player can't interpret, but the other characters in the game do.
In the Super Mario Bros. Super Show! Luigi is consistently portrayed as being the smarter of the two, but the cowardice trait is also shown as Luigi is scared of things going very wrong. He also doesn't take as many risks as Mario does.
Overall, Luigi's personality is hard to determine and tends to change depending on the appearance. Very few games have portrayed him with a personality that lasts throughout the subsequent release. Overall, however, Luigi can be described as timid and cautious, yet kindhearted and willing to protect his homeland with Mario's help - It is perhaps this irony, the basis of Luigi's character, that makes his personality so complex.