I have unlocked Roselina a long time ago and I don't remember how. She is the only character I have ever unlocked. I don't have any Mario Galaxy save, and I don't have the mirror levels, I don't even have all of the cups. I also have no stars for anything. How did I unlock her? Does anyone know?
According to http://mariokart.wikia.com/wiki/Rosalina
There are three ways to unlock Rosalina in Mario Kart Wii.
- Make a save file on Super Mario Galaxy, then perform 50 races.
- Get a minimum 1 Star ranking on all Cups in 150cc Mirror Mode.
- Play 4950 races.
Since you neither have a Super Mario Galaxy save file nor unlocked Mirror Mode, it seems you just played Mario Kart Wii a lot.