Princess Peach

Princess Peach is a prominent character in the Super Mario series and the Mushroom Kingdom's monarchical ruler. Bowser/King Koopa often kidnaps her. As a result, she often has to be rescued by Mario.

In Super Mario 64, Princess Peach's very own Mushroom Castle acts as the game's central hub world. Bowser has invaded the castle and kidnapped Peach once again, keeping her captive until the end of the game.

Behind the Stained Glass Window
Princess Peach's iconic stained glass window seems to hold some inconsistencies. The stained glass window also appears in the castle's foyer, although architecturally, its positioning wouldn't make sense in correlation to the castle's structure. Additionally, the stained glass texture inside the castle is different from the one outside the castle, including a darker purple background. If these are two separate windows, that must mean there is a room between the two.

In the game's ending, Peach appears in front of the stained glass window. A popular theory is that Bowser sealed Princess Peach within the room between the two stained glass windows.

This supposed room found behind the stained glass window is explored within one of Greenio's tapes, 01.22.96, and it is associated with the Internal Plexus.

Princess Daisy
Princess Daisy is the queen of Sarasaland and a recurring character in the Super Mario Bros. franchise, primarily appearing in spin-offs as a playable character. However, some have reported that Princess Daisy can appear in Super Mario 64 in place of Princess Peach, either replacing her entirely or appearing in her place in an Alternate Campaign. This commonly happens in conjunction with the Alternate Peach Letter Anomaly to fit the change of character. Whether or not she is legitimately unused to some extent or created by the Personalization A.I. through a process similar to Enemy Discoloring is unknown.

Trivia

 * An earlier version of Peach's model was found in the July 25th 2020 leak revealing that her model once had legs. These were likely removed in the final game as the cutscenes featuring her have her model go transparent, meaning her legs would be visible through her dress.