90's Kid

The 90's Kid is an anomalous entity that manifests within certain personalized copies of Super Mario 64. He is commonly found on the Second Floor of Princess Peach's Castle, and can either spawn as a playable character or an NPC. He appears as a teenager with brown hair, white skin, a red shirt with white stripes down the sides, blue jeans, and black and white shoes. Unlike other anomalous beings, his appearance stays relatively consistent from copy to copy, with only very minor changes in his model.

Behavior
As said before, the 90's Kid is usually seen running around on the Second Floor, using the various doors and talking to Toads. This behavior, however, comes to a full stop as soon as he spots Mario. He'll then approach Mario and talk to him in all caps, as opposed to other characters. Below is the most reported introduction phrase:

"HEY, MARIO!

I HAVE BEEN WAITING IN THIS ROOM TO MEET YOU!

I AM SO HAPPY TO SEE YOU!

MAY I JOIN YOU ON YOUR JOURNEY?"

The player would then be given two options: "YES" and "NO". If the player selects "NO", the character will immediately disappear and is never encountered again during the playthrough. If the player chooses "YES", the following text will appear:

"OH BOY!

THIS WILL BE SO MUCH FUN!"

After this, the kid will either become a fully playable character or an NPC that follows you. While in a level, he'll usually say jokes whenever Mario does something irregular, such as grab on a hangable ceiling and throw a Bob-Omb. The most commonly reported phrases are as follows:

"WHAT AM I? A MONKEY?"

"WOAH, BE CAREFUL WITH THAT!"

"I'VE NEVER SEEN A CINDER BLOCK LIKE THAT BEFORE!"

"AAH!"

Due to these being textboxes that commonly interrupt gameplay, it is advised that you select "NO" when he asks to join you. He is also unable to be damaged, as he always says the second phrase listed above. He can be killed by making him fall into an abyss or getting him burned by lava, though this doesn't get rid of him permanently. The only way to get rid of him permanently is by deleting the save file that has him.

Theorizing
Due to his humor and physical appearance, many players quickly found out that his manifestation is caused by watching this commercial before booting up the game, which adds the kid to the Personalization A.I.'s library of anomalies. This same method is theorized to work for getting other commercial objects to manifest, such as the milk jug from the "Got Milk?" commercial, though this is unconfirmed. There are also theories of other ways to make him manifest, such as having the A.I. detect that the player is lonely. There haven't been any reports of this, however.