Key Items

Key Items are anomalous items found mainly in Super Mario 64 meant to connect 2 or more games together via Stop N' Swopping to unlock rewards in one of the games. They typically appear once Stop N' Swop has already occurred due to the A.I. borrowing assets from one game and bringing them to another. They appear to be anomalous equivalents of the Ice Key and Stop N' Swop eggs from Banjo-Kazooie. Key Items often have no actual rewards due to the A.I. not creating the reward in the other game, either due to crossing over not effecting that game yet or some other reason, but real rewards have been well documented.

Notably, Key Items can sometimes take the appearance of well established anomalies that are completely separate from the Stop N' Swop process such as the Ghoul Medal or Big Bully's Key. This appears to be the A.I. recycling unused assets in a new way that previously was not observed until crossing over was discovered, as if these anomalies were created entirely by the A.I. it wouldn't make much sense to use it in completely different ways.

Ghoul Medal
The Ghoul Medal is one of the most common examples of Key Items. If one were to find the Ghoul Medal, it is said that it can be Stop N' Swopped into Paper Mario 64 where it is unlocked as a Badge. There are many different uses documented, but one of the most frequent is starting every battle with the effects of a Repel Gel for a few turns. Others include making Bow stronger, having a Boo possess the hammer and become a new partner, and making Mario completely invisible so that he can't attack or be attacked. The Ghoul Medal can also appear the other way around, usually somewhere in Bow's Mansion or Forever Forest in Paper Mario 64, and then be transferred to Super Mario 64. There, it can unlock a new level of some kind related to ghosts and Boos or Paper Mario as a whole, give the player the effect of the Vanish Cap permanently with the ability to turn it on and off, or simply appear on the player's save file.

Big Bully's Key
Big Bully's Key can reportedly appear as a Key Item applicable to both Banjo-Kazooie and Banjo-Tooie. As one would expect, it is acquired the same way it's found when it's exclusive to Super Mario 64, and appears to be intentionally set up as an analog to the Ice Key from Banjo-Kazooie. In Banjo-Kazooie, some say Big Bully's Key could unlock turning Kazooie into a dragon like seen in the XBox Live Arcade port of Banjo-Tooie or the cut Mount Fire Eyes level, and in Banjo-Tooie a new lava level would be unlocked. If it appears in Banjo-Kazooie or Tooie, typically in a lava level of some kind, it usually retains it's purpose of unlocking Lethal Lava Ruins in Super Mario 64.

Ocarina of Time
The Ocarina of Time can reportedly appear in Super Mario 64, commonly somewhere in an anomalous temple-esque level of some kind of Princess Peach's Castle. According to those who find it, the player has to go to the Starry-Night Skies painting and play the stars on it as notes, playing the Song of Storms. This can reportedly cause some clouds to appear in Hyrule (Legend of Zelda: Ocarina of Time) with the Sky Temple. Another song can reportedly be played at the Eternal Star Statue, usually the 1-Up jingle, Bob-omb Battlefield's theme or World 1-1's theme, that can unlock the Mario mask seen on the Happy Mask Salesman's backpack in Legend of Zelda: Majora's Mask that allows the player to jump.

Coconut Gun
According to some, Donkey Kong's Coconut Gun from Donkey Kong 64 can appear in Super Mario 64 that works similarly to how it does in the original game it came from. This can reportedly allow the player to break walls or new objects they previously couldn't to find something or complete some kind of quest to unlock something in Donkey Kong 64, usually an upgraded Coconut Gun of some kind or a new level appearing.

Warp Star
One of 2 common Key Items for Kirby 64: Crystal Shards, some say that in Super Mario 64, a random Power Star will be replaced with a similar object that appears to be the Warp Star from the Kirby franchise. Upon being collected, it either directly allows the player to unlock new content in Kirby 64 or warp the player to a Kirby 64-themed level of some kind that upon completion would then allow the player to Stop N' Swop. This can reportedly unlock the unused whale level seen in concept art, or Adeleine, King Dedede, and/or the Waddle Dee as playable characters.

Strawberry Shortcake
The other common Key Item for Kirby 64, some say that one can find a unique cake resembling the kind of cake seen in the Kirby franchise, usually in Delicious Cake or the Castle Kitchen, that, like the Warp Star, can either unlock the aforementioned levels it can also be located in or directly unlock new content in Kirby 64, which is usually the same content as the Warp Star.

Dice Boxes
According to those who have encountered this Key Item, ! Boxes, usually yellow ones, can randomly be replaced with dice boxes seen in the Mario Party games that give the player 1-Ups or coins, with the amount depending on the number you rolled. If you get the maximum number available, a smaller pair of dice can be dropped that can then be Stop N' Swopped to any of the 3 Mario Party games on the Nintendo 64. These usually unlock new boards or minigames of some kind, though the exact boards or minigames are never consistent.

Race Shells
A type of Key Item that appears to go hand-in-hand with Enemy Discoloring, discolored Koopas can drop shells that go faster than normal and don't disappear upon being crashed. They can usually be used in some kind of extra Koopa the Quick race or to break through some kind of barrier to reach a new area that would then allow the player to Stop N' Swop with Mario Kart 64. This usually unlocks a new racer such as the Magikoopa or even Marty the Thwomp, or a unique track such as Pokey Oasis.

Majora's Mask
Majora's Mask can reportedly appear in Legend of Zelda: Ocarina of Time upon crossing over effecting the cartridge, commonly in the Happy Mask Shop. Some say that if you buy every mask from him after completing the mask trading sequence, you can buy Majora's Mask from him and unlock Fierce Deity Link in Ocarina of Time, albeit for a limited time before it wears off. According to those who then Stop N' Swopped with Legend of Zelda: Majora's Mask, an adult Link mask would then be unlocked, allowing the player to play as adult Link from Ocarina of Time in Majora's Mask, which is usually impossible.

Theorizing
It is believed that Key Items are entirely constructs of the Personalization A.I., as Stop N' Swopping was a process developed by Rare and not Nintendo, who actively discouraged it from being used. How the A.I. learns about the other games is confusing, as sometimes it can reference games that have not been effected by crossing over. However, through the process of Inverse Swapping, one may accidentally allow the A.I. to access assets from a game that would then be used to create a Key Item. This is corroborated by the fact that Key Items seem to only reference games that the player already has. Alternatively, the A.I. could use data from the Controller Pak or even the unused saving feature seen in the scrapped Nintendo 64 BIOS.