To unlock Rosalina, you have to complete the Star Cup in Grand Prix mode. The Star Cup is available from the start of the game, so you can start there immediately if you so choose.

In a little bit of a surprising move, Mario Kart World doesn’t provide you with all playable characters right from the starting line. Household names like Donkey Kong, Daisy, Rosalina, and more have to be earned by meeting set conditions within the main campaign,

You can obtain Mario Kart World’s unlockables through various methods, so here's how to get new stickers, karts, characters, and costumes.