How to unlock characters in Mario Party Switch
Step 1:
Play through the Party Mode until you complete the first world and unlock Toad as a playable character.
Step 2:
Complete the second world to unlock Diddy Kong.
Step 3:
Win a certain number of mini-games to get Bowser Jr. to join your party.
Step 4:
Play through the game and complete the Moon Rock Hunt to unlock Rosalina.
Step 5:
Complete the River Survival Mode to unlock Donkey Kong.