After you've completed the requirement, head over to the Party Plaza and look for the character hanging out in the background. Speak with them to unlock them!
- Donkey Kong - Finish three different courses in the River Survival mode. Keep an eye for him out in the crowd at the end of the river! (Dice: +5© 0 0 0 10 10)
- Diddy Kong - Complete World 2 (Chestnut Forest) in Challenge Road. (Dice: 0 0 0 7 7 7)
- Dry Bones - Will randomly appear in the Party Plaza. (Dice: 1 1 1 6 6 6)
- Pom Pom - Complete World 5 (Salty Sea) in Challenge Road. (Dice: 0 3 3 3 3 8)
Reddit user Trilerium did the math and compiled it into a very helpful table here. In the table at the link, and below, you can see the mean dice roll and how high you average roll with characters like Bowser, Boo, and Wario can be. Note that these characters also come with the risk of losing coins AND not moving. But on average, they should generally get your further than any other character.
ncG1vNJzZmivp6x7qrPNZ5qopV%2Bstqy10miqrqiVp3qurdGipmaokafBunuhnqqtl3OdrrOtwq2cq6uPlrulq6Oimp4%3D