R.O.B is actually the first crossover character of the main Mario Kart games: none of its games involved Mario.
This is also the reason why it isn't currently likely to appear, as the other crossover content is missing as well in Mario Kart Tour (which is something I actually like, to be fully honest).
This is also the reason why it isn't currently likely to appear, as the other crossover content is missing as well in Mario Kart Tour (which is something I actually like, to be fully honest).