Rosalina-X is the dark and evil version of the Super Mario Galaxy Rosalina.
Rosalina-x was a distant idantical twin of Rosalina. When she was born she was accidentally chucked off a 60-mile-high Cliff. While falling her clothes were burnt during atmospherically friction, turning her clothes "Midnight Black". The Friction also made Rosalina-X indestructible and gave her special powers and abilities like flight, invisibility, teleportation, fireballs, tornado, attraction, and more.
When Rosalina-X crashed at he bottom of the cliff, she was already an attractive teenager. Rosalina-X never really crashed at the bottom of the cliff, she really landed softly using her special abilities. At the bottom she landed right next Metal Princess Peach completely by coincidence. Finding their simularities, evilness, and special abilities, they became evil BFFs/partners for life dedicated to destroy Mario and Luigi.