It’s not easy being Luigi. The lesser of the two Mario brothers constantly plays second fiddle to the more famous Nintendo mascot Mario, a fact that has become so true over the years that some of their games actively poke fun at it.
The ‘skinny one with the green hat’ is constantly presented as the shyer, more timid of the two. Indeed, the original reason pertaining of his creation was the need of a player two character in Nintendo’s games.
His somewhat paranoid nature has influenced him to stay behind at home while letting the elder of the siblings save the Princess and her kingdom.
Luigi’s first appearance was in the Game & Watch Mario Bros. (released several months before the more-popular arcade Mario Bros.) game.
Super Mario Bros. Series
Critically, no difference existed between him and his brother, until Super Mario Bros 2 divided their abilities. Luigi was deemed the less powerful of the two, yet possessing a more advanced jump. The Mario team in Japan considered this to be an error unique to Mario 2 (which they called Super Mario USA), and actually kept Luigi as a standard palette swap in Super Mario Bros 3 and Super Mario World. Nintendo didn't acknowledge the brothers as 2 different people again until much later.
Unlike the many rumors about playing as Luigi in Super Mario 64, this one is actually true. Interestingly, Luigi actually did become a playable character in the handheld remake, Super Mario 64 DS.
In Super Mario Galaxy, you can unlock Luigi by getting all 120 stars with Mario and then beating Bowser again.
Paper Mario Series
Other escapades by the little green man are told rather than played - the second iteration of Paper Mario involved a humorous side story running parallel to Mario’s own adventure.
As a core character in Nintendo’s franchise, Luigi makes regular appearances in Mario’s spin off titles, such as Mario Party and Super Smash Bros..
A handful of games have starred Luigi, despite his reluctance to go adventuring. The well-known edutainment title Mario is Missing! was not developed by Nintendo’s core team, and so was somewhat lost in the familiarity of the Mario universe. In fact, the GameCube title Luigi’s Mansion ignored the existence of Mario is Missing! and described itself as Luigi's first starring role, and was much better received, allowing the taller brother to finally shine. The irony that both of these titles involve rescuing Mario seems not to be lost on the game’s creators.
Supposedly his greater athletic ability led to him becoming a taller and slimmer size than his better known pasta loving brother.
Baby Luigi is the baby version of Luigi that usually gets kidnapped in Super Mario World 2: Yoshi's Island. During his appearance, Yoshi and Baby Mario traveled all across the island to save him from Kamek and Bowser. After that, the babies are reunited with his parents. Baby Luigi has since appeared in many Mario spin-offs.
In the minigames in New Super Mario Bros. and Super Mario 64 DS, Luigi is a card dealer and you can play games such as Luigi-jack (actually blackjack with a different name), Mushroom Roulette, and Picture Poker. He also appeared in all 91 episodes of the Mario cartoon series.
In Super Paper Mario, Luigi appears as the "mysterious enigma" known only as Mr. L. He commands the Brobot L-Type in battle. The player can obviously see that he is Mario's brother Luigi, but the slow-witted Mario seems surprised to discover this.
Several games seem to imply that Luigi is romantically linked to Princess Daisy, if for no other reason than to give a counterpart to Mario's feelings for Peach.