Persimmon
From Starfy Wiki
The Kaki (Japanese: カキ) is an item in Densetsu no Starfy 3, found in Old Castle (Stage 10-1). It is not available on the Items List. While its name means 'oyster', it looks more like an orange fruit so it is unclear what it actually is. From a Shell Tape, Starfy, Starly and Moe find out that a special item is required to open the large door to Old Castle and Starly suggests that the item may be a Key. Instead of a Key, Starfy finds the similar sounding Kaki and gives it to Starly but Starly throws it back at Starfy apparently thinking it is no good. On the contrary however, the Kaki lands in the statue above the large door's mouth and the doors open.