Bowser Keys or Keys are items used to progress through key doors that are obtained by defeating Bowser at the end of a Bowser Stage. There are two keys in Super Mario 64, one from Bowser in the Dark World, which unlocks the basement, and one from Bowser in the Fire Sea, which unlocks the second floor. The only way to skip a key is to do Moat Door Skip to go into the basement without ever having to interact with the basement key door. There is no way to skip the upstairs key door since it triggers the loading zone to the second floor, with no difference entrances to get in (e.g., moat door). Because of this, Bowser in the Fire Sea is mandatory to finish the game.
Mario with his head pointing down and star over his head after picking up a key on the Japanese version.