Club Key
From Starfy Wiki
The Club Key (Japanese: クラブキー) is green key in Densetsu no Starfy. It's found in the 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 other keys are the Heart Key, the Spade Key and the Diamond Key. They are all based on the four French playing card suits.
It's found through the top-right door of the main room (with the prison cell). When Starfy reaches the deck of the ship he finds the Club Key, only for a Kagiban to swallow the key and try to flee. Starfy must hit it three times with a Star Spin to defeat it and leave behind the key.