Princess Daisy

Princess Daisy is a character first introduced in Super Mario Land for the original Gameboy. After which she only reappeared in spin-off titles such as the Mario Party and Mario Sports series. (With the small exception of Super Mario Run, which saw her kidnapped once again as Mario set off to rescue her.) After Super Mario Land, she has most commonly been paired with Luigi rather than Mario.

Daisy makes no appearance in Super Mario 64, or its DS Remake. However reports have stated that Princess Daisy has been seen in personalized versions of the game, most notably the Luigi Alternate Campaign, where she replaces Princess Peach. However she has been sighted in versions of the game without the Alternate Campaign personalization, yet again replacing Peach.

Behavior
Princess Daisy, whether she be in the Luigi Alternate Campaign or a normal playthrough without said anomaly, behaves identically to Princess Peach, even sharing her exact voice clips. The only differences are aesthetic, with Peach being re-colored to fit Princess Daisy. Specifically the version of her introduced in Mario Tennis, which notably shares some assets with Super Mario 64.

Theories
Theories for Princess Daisy’s appearance in personalized copies of Super Mario 64 mainly link back to similar theories for Luigi’s appearance as a playable character, and the manifestation of the Wario Apparition. Some players have a subconscious desire for certain characters to appear in the game, and if the Personalization A.I. is able to detect this through their gameplay it will try to compensate. With Daisy being considerably easier to implement than Luigi and Wario (One being a recolor, while the other two are an entirely new playable character and a supposed scrapped boss fight) it’s a wonder why Daisy does not appear in more personalized playthroughs. This could be chalked up to Daisy simply not being an immensely popular character, at least when compared to others who have appeared in personalized copies.