Stanley

Stanley, generally shortened to Stan, is a manifestation of the Personalisation A.I. and a rare anomaly within Super Mario 64. Taking the form of a textureless Mario with very pale skin and a yellow cap and shirt, Stanley can be found within Princess Peach's Castle, roaming around and mimicking some of the player's movements. A player can interact with him. However, it's advised not to, as interacting with Stanley will cause a permanent corruption to the player's copy of Super Mario 64, forcing persistent anomalies and disturbing imagery.

Theorising
The creation of Stanley might be because the AI wants to play with the player. Another possibility is that the AI is testing the player skill and is adapting to them.

Trivia

 * Stanley is a name and identity given by the MIPS Hole Discord group. It's possible that with the A.I.'s advanced learning that it adopted Stanley as its way of manifestation.
 * Stanley is also the name of another Mario character, Stanley the Bugman, who appears in Donkey Kong 3.
 * A copy of the original Stanley article has been pasted here.