As an N64 launch title, Super Mario 64 shepherded Nintendo into the 3D era and became massively influential, with it still regarded as a platforming masterpiece over two decades later. The game has players control the eponymous plumber in an attempt to save Princess Peach from Bowser. The story was not new to the Mario games, and while it featured many familiar sights of the Mushroom Kingdom, there was a noticeable absence from the game: Mario's twin brother Luigi, the other half of the Mario Bros.
For years, fans thought Luigi was secretly accessible in Super Mario 64. One of gaming's longest running conspiracy theories cropped up surrounding an in-game plaque that appeared to read "L is real 2401." Many took this as a sign that Luigi could somehow be unlocked, with the most popular answer to the cryptic number in the message possibly being a reference to the total number of gold coins throughout SM64's levels. Super Mario 64's source code leaking actually proved the conspiracy to be true in a roundabout way - Luigi's character model did indeed exist within Super Mario 64 at one point, but there was no way to play as him without modding the game.
The actual reason for Luigi having a character model without being playable in Super Mario 64 is much less exciting: hardware constraints. According to Looper, legendary Mario creator Shigeru Miyamoto actually addressed the lack of Luigi in SM64 way back in 1996 when it released. Super Mario 64 was initially planned as the next Super Mario Bros. game and would feature both of the brothers, but due to "memory issues" Luigi had to be axed. Luigi would not join his brother in the 3D game-scape until he was included in Mario Kart 64.
Luigi Eventually Made It Into Super Mario 64
Luigi would have to wait eight years in order to officially be a part of Super Mario 64, albeit in a remake. In 2004, Nintendo released Super Mario 64 DS, a remake of the platforming classic for the launch of the two-screened handheld device. The DS remake featured not only Luigi, but four playable characters in total - Mario, Luigi, Yoshi, and Wario. Unfortunately Luigi still wouldn't get the limelight, since Yoshi was made the de facto protagonist in Super Mario 64 DS, arriving late to Peach's castle and having to find the other three playable characters within.
Luigi remains Mario's most faithful companion, but Luigi as a playable character being excluded from Super Mario 64 shaped his future in the 3D platforming franchise. Mario remained the singular star in Super Mario Sunshine, Galaxy, Galaxy 2, and Odyssey. He still remains the default number two in games that involve multiple players, and he's had his own success with games like Luigi's Mansion, but Mario's revolutionary role in Super Mario 64 only increased the already growing gap between the two brothers' stardoms.
Source: Looper