Rosalina was one of Elsa the Snow Queen's clones. However, Elsa hated that despite having virtually the same personality, Rosalina looked NOTHING like her. So, Elsa banished her to the cosmos.
Rosalina is the queen of the Cosmos. She lives in an ice palace much like Elsa's, but instead of having a snowman named Olaf as a companion she lives alongside this star thingy called Luma.
She then sees an infamous troll on YouTube part of the Galactic YouTube Trolling Crew known as MarioTehPlumber who obsesses over her and claims she's "his waifu". This is weird, though, so she never talks about it.
Peach hates her, and Daisy is planning to kill her.
She loves stars.
She killed every last Adele Penguin.
She breaks out when she sees people pairing her with Waluigi.