Waluigi

Waluigi is a popular character appearing in many spin off Mario titles, such as Mario Tennis and Mario Kart. In Super Mario 64 DS, his absence from official artwork and mostly no presence in the game led many to believe that he was either scrapped from the game or really well hidden, similar to that of Luigi in the original Super Mario 64.

Some people, such as popular Instagram User and creator of The Film Bro saga "CiinemaGunner," have found him within the game. The method varies from person to person, leading some to believe that Super Mario 64 DS has a personalization AI in the game, as the original did.

Gunner's Method
The Method Gunner had demonstrated in his now defunct Instagram account was as follows. First, you had to have every star and every minigame for this to work. This was to ensure that when Waluigi's Mini-Games were uncovered, it would not crash the game with other locked Mini-Games, as this was intended to be unlocked after everything else.

From here, you would use Wario to get the metal cap on the castle roof and blast away the wall where the black box on the file select screen appears on the castle. It will reveal a door that you can enter to find Waluigi's painting. The course is known as "Count Von Koopa's Challenge"