From the Super Mario Wiki
Starborn Valley is the place where Star Kids, like Twink, are born in the game Paper Mario. Merlon's son, Merle lives here and, with help from a group of Ninji, raise the Star Kids before they leave for Star Haven. While Mario and his partners visit this place, they learn from Merle that the last of the Star Spirits is being held in the Crystal Palace. To get to Starborn Valley Mario needs to trek across Shiver Snowfield. Frost T. as well as other Toads also lives here.
Before the player reaches the valley, Monstar appears and fights Mario, but turns out to be made of the stars here who only wanted to self-defend. Merle later apologies for the trouble the stars have caused. The in-game map description reads, "Star Kids are born in this sparkling valley. Frost T. and Ninjis take care of the newly born Star Kids. Merle's house and a Toad House are here, too. "
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