Subspace Emissary Key.

A Key is an item in Super Smash Bros. Brawl, specifically in the Adventure Mode: The Subspace Emissary. True to their nature, they are used to unlock various doors encountered in the mode. They resemble the Small Keys in Zelda: Twilight Princess. If the player loses a key before they could reach the door, just return to where you originally found it. They can also be thrown, and do some damage.

Trophy Info

In The Subspace Emissary, there are often locked doors. This key is the item you need to unlock these doors. You can touch a door while holding the key, or you can even throw the key at a door to unlock it. If you lose the key, it will return to the place you originally found it. This item is vital to your progress, so do your best not to lose it.