Right off the bat it has no eyes, no ears, and no mouth. A trait only shared with the Regis. Its "face" is instead dominated by a single red gem. Despite the fact that it is clearly a Starfish it is identified as the, "Mysterious Pokemon."
Some Pokedex entries:
Regardless of the environment it lives in, its body grows to form a symmetrical geometric shape.
People in ancient times imagined that Starmie were transformed from the reflections of stars that twinkled on gentle waves at night.
This POKéMON has a geometric body. Because of its body, the locals suspect that it is an alien creature.
Its core shines in many colors and sends radio signals into space to communicate with something.
Crazy Fan Theory:
Many centuries in the past the ancient Starmie species was transformed by a an extra-dimensional organism as it made contact with them. Unfamiliar with concepts of curves and smoothness, it crafted the bodies of the Starmie into radially symmetrical simplifications of the their original forms. Every night they are able to, the Starmie transmit information back to their creator that they have picked up via telephone communication/radio waves/and other wireless communication.
They act as the a beacon and guide the "something" as it searches for a rift where it can fully emerge into this dimension.