Waluigi

Waluigi is a recurring character found in the Mario series who was made to rival Luigi. Waluigi mainly appeared in spin-off games like Mario Tennis and Mario Party 3. Waluigi is known to be Wario's partner ever since Waluigi made his initial appearance in Mario Tennis for the Nintendo 64. The details of Waluigi's past are unclear since he has no confirmed background, although he has evidently been antagonizing the Mario Bros, especially Luigi. In his debut as seen in the opening of Mario Tennis, where he and Luigi confront each other, it is implied that they have been fighting for quite some time.

Despite the appearance of all three main plumbers in Super Mario 64 DS, Waluigi was nowhere to be found. And it is unknown why, considering how a popular Nintendo-related magazine had a tutorial in one of its numerous issues on how to unlock the purple plumber (Though it may not have been official).

Unlocking Waluigi
Several members of the Super Mario community have speculated about a possible way to unlock the lanky plumber in Super Mario 64 DS. However, there isn't a clear way to do so. The most prevalent method to "unlock" Waluigi was the Purple Prizes, a magazine which claimed Waluigi was unlockable. The instructions are as follows:

It claims only the best players could find the character. He could be unlocked after gathering all the stars and "being the fastest foot racer in the land". Achieving this, the secret door would unlock. The player would have to race the Rabbit King, who is supposedly the leader of all the rabbits found inside the game. The article also claims that Waluigi's special ability is 'Spring Jump', and that his power flower allowed the player to run extremely fast. This was clearly fake, and no evidence of Waluigi being in the game or being ever planned for the game was uncovered.

Despite no traces of Waluigi being in Super Mario 64 DS, a purple rabbit can be seen in the US instruction manual for the game. While this could be chalked up to poor printing of the pinkish rabbits belonging to Mario, this rabbit's coloration is distinctly purple hinting that these could've belonged to Waluigi at one point. Additionally, it's rumored that files relating to Waluigi or the areas associated with him can be found in the PAL version of Super Mario 64 DS.

Super Mario 64 Manifestation
There have been reports of Waluigi in their personalized copies of Super Mario 64, rarely as a playable character. They have been reported to run around the castle, often in the exact opposite direction of the player. It is believed this is a remnant of Wario Land 64, where Waluigi would have appeared. This doesn't make sense however as the concept of Waluigi himself wasn't created until 2000 so takes these claims with a large amount of salt