Luigi is portrayed as the taller, younger brother of Mario, and is usually seen dressed in a green shirt, dark blue overalls, and a green hat with a green "L" insignia. Although Luigi is a plumber like Mario, other facets of his personality vary from game to game; Luigi always seems nervous and timid, but is good-natured and can keep his temper better than his brother. A baby version of the character named Baby Luigi debuted in Super Mario World 2: Yoshi's Island, who is held captive by Kamek. He also appeared in Mario & Luigi: Partners in Time as a playable character along with Baby Mario. He is voiced by Charles Martinet, just like his adult self. According to Miyamoto, Mario is 24 years old, indirectly confirming Luigi is 24 as well.

While it has not been made official, Daisy has been rumored to be Luigi's romantic interest. In Mario Kart Wii they are seen in statue dancing together. She was his caddy in NES Open Tournament Golf, as Peach was to Mario. On Daisy's trophy in Super Smash Bros. Melee, it says that she is possibly Luigi's answer to Mario's Peach. Nintendo did not initially give Luigi a surname. The first notable use of "Luigi Mario" was in the 1993 live-action film adaptation. In September 2015, at the Super Mario Bros. 30th Anniversary festival, Miyamoto stated that Mario's full name was Mario Mario. As a result, this indirectly confirms Luigi's full name to be Luigi Mario.

Luigi's first appearance was in the 1983 arcade game Mario Bros. as the character controlled by the second player. He retained this role in Wrecking Crew. He later appeared in Super Mario Bros. for the NES, and again in Super Mario Bros.: The Lost Levels, Super Mario Bros. 2, Super Mario Bros. 3, and Super Mario World. Super Mario Bros. 2 introduced Luigi as the taller of the two brothers, as well as the better jumper. Super Mario Bros. 3, and Super Mario World returned to featuring Luigi as identical to Mario. He made a minor appearance in his baby form in Super Mario World 2: Yoshi's Island. Luigi was conspicuously absent in Super Mario 64 and Super Mario Sunshine. However, the Nintendo DS remake of Super Mario 64 features him as a playable character alongside Mario, Yoshi, and Wario.

Luigi has been associated with the more difficult second acts of multiple Super Mario games. These include The Lost Levels, Super Mario Galaxy 2, New Super Luigi U and the new game plus in Super Mario 3D Land, which offer more challenging elaborations on their respective predecessors and allow the player to use Luigi as the main character, with whom reduced friction and higher jumping is consistent in all of these games. Luigi became playable in the Nintendo DS game New Super Mario Bros. as a hidden character, and as a hidden character in the Wii game Super Mario Galaxy. In its sequel, Super Mario Galaxy 2, the player can switch out for Luigi throughout the game. He also appears as a playable character in New Super Mario Bros. Wii, where four players can play at once cooperatively as Mario, Luigi, and two Toads. He also appears in Super Mario 3D Land as a playable character as well as New Super Mario Bros. 2 and New Super Mario Bros. U, the latter having a DLC mode, where he is the main character, called New Super Luigi U. It has levels altered to his specific play abilities, including higher jumping. The DLC is also available as a standalone retail version. Luigi also appeared in Super Mario 3D World along with his brother, Peach, Rosalina and Toad.

Luigi appears in many of the Mario spin-off games, including Mario Kart, Mario Party, and all of the Mario sports games. He also appears in all five installments of the Super Smash Bros. series; in the first three installments and Ultimate, he is an unlockable character. Luigi received his own starring role in the 2001 video game Luigi's Mansion, where he wins a mansion from a contest he never entered, and saves Mario from King Boo. He reprised his role in the installments Luigi's Mansion: Dark Moon and Luigi's Mansion 3.

Luigi has appeared in every Mario role-playing games. While he originally made a cameo appearance in the end credits of Super Mario RPG, he appears more prominently in the Paper Mari…

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Allegra Frank of Polygon commented that the English manual for Mario Party described him as "smarter than Mario", but since Luigi's Mansion, Luigi has been defined by his wimpish persona, and due to being overshadowed by Mario, he "has an inferiority complex unparalleled in gaming". Den of Geek writer David Crow thought Luigi was "cooler" than Mario because he "jumps higher, runs faster", and particularly praised his appearance in the 1993 Super Mario Bros. movie as portrayed by John Leguizamo. Alex Siquig writing for The Ringer also felt that Luigi was unappreciated. He commented that Luigi has existed "within the confines of Mario’s pixelated shadow since 1983" but felt that he had successfully carved out his own personality making a relatable character that is more than just a palette swap for Mario.