From the Super Mario Wiki
This young Star Spirit is pure hearted but strong. She is one of the two female Star Spirits and wears a yellow ribbon on her head.
Misstar was rescued by Mario and his team on Lavalava Island. She was guarded by the aggressive Lava Piranha and his Lava Buds. After Mario rescued her, Misstar (in turn) helped Mario, Kolorado and Mario's team escape the volcano that was about to erupt. She added her powers to that of Mario's and left to meet up with the other Star Spirits.
She is also found in Mario Party 5 hosting one of the game modes. She hosts the story mode. In this game, she only has beady eyes.
A Star Spirit that looks like Misstar appears in Mario & Luigi: Partners in Time. In Star Hill, a Star Spirit with a mouth and a ribbon that always flows appears and tell the Mario Brothers and Baby Brothers to gather all the Cobalt Shards. However, it is revealed that she was not a Star Spirit and was actually Elder Princess Shroob in disguise.
“She's the Star Spirit, Misstar. She was imprisoned by Bowser in Mt. Lavalava. Her life must've been really tough recently. But she still looks pretty. ♥♥♥ Boy, she makes me see stars!”
 Misstar's Power
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