From the Super Mario Wiki, the Mario encyclopedia
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The Luigi Key is an item in Super Mario 64 DS. It is the second character key to be retrieved in the game. The Luigi Key was used by King Boo to trap Luigi behind a door in the Mushroom Castle after he captured him. In order to obtain the key, Mario must get into a secret room in Big Boo's Haunt by using a Power Flower and navigate through a maze to find King Boo and defeat him.
The Luigi Key is made of gold with an L in the center of it, as well as Luigi's cap on top of it. Once the player unlocks Luigi's door with this key, Luigi becomes playable and his caps will appear in courses.