Luigi

Luigi is one of the many characters from the Super Mario series, being the cowardly brother to the heroic Mario. He often fills the role of player two in most of his appearances and is infamously portrayed as living in his brother's shadow.

Despite Luigi appearing in almost every mainline Mario game beforehand, Super Mario 64 is the fifth game to not show any signs of Luigi. While Luigi was originally intended to be in Super Mario 64 alongside its multiplayer mode, he was removed when the mode proved too troublesome for the developers. Despite this, Luigi's influence has seeped into some personalized copies of Super Mario 64.

Removal
Luigi was seemingly "removed" from the game in February 1996 due to "memory limitations." Should he have remained in the game, and levels would have to be much simpler to compensate. However, the team wanted to include him in a minigame similar to Mario Bros. However, this mode was cut due to the N64 only being sold with one controller.

Remnants
There are multiple remnants of the scrapped two-player functionality within Super Mario 64. This includes duplicates or empty pointers proceeding directly after Mario's pointers, and an unused camera intended to keep focus on two active objects.

Perhaps the biggest remnant revolving to Luigi can be found within the iQue leak, where the remnants of Luigi's model could be found within the folder attaining to the player. This included colors matched to Luigi's model, and three textures specifically for Luigi. Upon its discovery, users got to work to assembling Luigi's model.

Eternal Star Statue
In the castle courtyard is a fountain with a large star statue in the middle. The star statue has a plaque underneath it, which has vague writing etched onto it. The plaque can barely be read, but much people reads "L is Real 2401." Many people speculated this was actually Paper Mario’s release date in North America (February 5th, 2001) however, this remains unconfirmed as well as the fact that the games were made by 2 separate teams and most likely at different times alongside the fact that the “L is real" part has very little to do with Paper Mario leads many to dismiss this as coincidence. This texture is also used in The Legend of Zelda: Ocarina of Time for the epitaphs on the gravestones, perhaps linking the AI to The Legend of Zelda series as well.

The Luigi Unlock Theory
An anomaly that remains uncertain is the definitive way to unlock Luigi. Many people have came up with claims on how to unlock Luigi, most revolving around the Eternal Star Statue, and others referring to game completion bonuses. While this should be taken with caution, there is a theory that says Luigi's unlockability may be randomized based on Nintendo's AI. This could be an explanation for the many rumors on how to unlock him because of the A.I. potentially reincorporating the unused files to reintroduce Luigi to the player.

Luigi Mechanic Tweaks
Similarly to Wario, Luigi can reportedly have a variety of abilities in Super Mario 64 DS that are not in the base game. He can dive on water and bounce a few times like a skipping stone before sinking, walk on the seafloor (without the ability to press switches due to his decreased weight), and slide and dive on land similarly to Super Mario Sunshine. Why exactly these abilities manifest is unknown, unlike Wario's abilities which make much more sense.

Trivia

 * When the iQue leak was circulating on July 25th 2020, a set of voice samples were circulating on the internet supposedly belonging to Luigi in Super Mario 64. However, this is merely false as no files retaining to Luigi's voice could be found in the leak. These voice clips originate from Super Mario Advance.
 * Super Mario Advance was one of the game's featured in the Nintendo Gigaleak, meaning that it's likely that these voice clips were arrogantly paraded around as from Super Mario 64.