Treasures are collectible items found throughout each Starfy game since Densetsu no Starfy. The treasures are found in the extra levels after beating the game in Densetsu no Starfy 1 - 4, but in The Legendary Starfy, they can be found during the main game. They've always contributed to encountering the true final boss post-game.
Starfy is seen carrying a treasure chest, a crown, and the jar that contains Ogura. Starfy tripped and fell, dropping the items, with the jar falling off of the edge. Ogura escapes from his jar and causes a thunder storm. The storm causes Starfy to faint and fall into the sea, beginning the game's storyline. After winning the main game, Starfy must find 44 treasures in order to access the Extra Final Boss. He gets the 45th treasure from him. Each treasure adds an icon to the Treasure Items panel.
Unlike Densetsu no Starfy, in the main story, there are treasure chests at the end of each level which contain Stuff. After completing the game, there are additional treasures in the extra levels that add to the Treasure Items panel. Once Starfy finds the 44 additional treasures, he can get the last one by defeating Ogura.
Instead of the usual treasure chests, Starfy and Starly must find the Evil Crystals. After the story is completed, the player can fight Evil, but he cannot be defeated entirely unless all the Evil Crystals have been found.