From the Super Mario Wiki
The Starfish is an elusive enemy, appearing only in Super Princess Peach. It is a reference to the video game character Starfy, as the glossary describes him as a star-shaped foe rumored to be the prince of a kingdom which Starfy is, as well as running, spinning, and swimming the same as him.
It has pink cheeks and wears black sunglasses. This is a reference to Starfy's hobby of wearing and collecting sunglasses.
Although the Starfish neither attempts nor is able to harm Princess Peach in any way, the game describes it twice as an enemy. In the game's glossary, it is described as "a star-shaped foe[...]". Also, wherever it can be found, there is a message box stating it to be an "odd enemy".
The Starfish, out of all the enemies in the game, is the hardest enemy to find and defeat. It appears in a specific point at each of three levels: 2-5 in Hoo's Wood, 5-5 in Wavy Beach, and 6-4 in Gleam Glacier. If the specific area that it can be found in is entered, there is a low chance that it will be there. If it is not there, players can leave the place via a pipe and immediately come back, for another chance of it being there. To defeat it, players must attack it in each of these three places.
In Hoo's Woods, it swims back and forth in a shallow area of water, and eventually will jump up into the sky and leave. Players must hit it as it jumps up.
In Wavy Beach, it is asleep on the grass. The player must walk slowly up to it, then attack it, so it doesn't wake up.
In Gleaming Glacier, it stands on a ledge above the player. It will leave after a few seconds. To reach it, players must use their Gloom vibe.
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