The Red Key is an item in Paper Mario: The Thousand-Year Door.
In Paper Mario: The Thousand-Year Door[]
The Red Key is located in The Great Tree, where it is held by an X-Naut in the storage room. After Mario enters the storage room, the X-Naut becomes conscious, mistakes Mario for hitting him, and a battle starts. The X-Naut drops the Red Key upon being defeated. With the Red Key, Mario can unlock the cage the Puni Elder is imprisoned in.