Only in Japan!
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The Spade Key (Japanese: スペードキー) is a blue key that is seen in both Densetsu no Starfy and Densetsu no Starfy 2. It is one of four special keys. The other three are the Heart Key, the Club Key and the Diamond Key. They are all based on the four French playing card suits.
Densetsu no Starfy
In Densetsu no Starfy, the Spade Key is found in Sunken Ship. It was originally under the possession of Boctopus. It is one of four keys required to open Sunken Ship's prison cell and rescue Dolchina and Dolshi.
The Spade Key is found through the top-left door of the main room (with the prison cell). Through this door, Starfy must navigate a room of strong currents. Starfy can pass through the currents with the Mighty Star Spin, though there are taps to temporarily remove the currents when they are hit with a Star Spin so that the player can navigate the room more easily.
Densetsu no Starfy 2
In Densetsu no Starfy 2, the Spade Key is found in Ogura Castle (Stage 11-4). It is one of four keys required to unlock the blue cage Moe was trapped in after he fell for an illusion of Ruby created by Ogura. It is found through the bottom-left door. Starfy must navigate a room with Ryun.