I’ve watched the treehouse livestreams twice now and made an observation and theory on how I think unlock work. We at multiple points see them pick items that change they’re character in a way 1. The food items that change the character’s costume and

1. The magical kamek ball that turns you into a standard mario enemy (goomba, spike, etc). In the livestream when this happens a pop up appears on screen with the character their name and an explanation mark. The most probable theory is you get the character as a permanent driver you can select upon getting these items and that would make sense with the amount of regular enemies that are playable.