Super Mario Party Jamboree has two secret characters you can unlock, and despite what you might think, they are pretty easy to come across. They won’t require any grinding or achievement scoring to add to your roster and can be found pretty sharpish.
These two characters are Pauline and Ninji, who are making their Party debuts and appear heavily in the game’s advertisement but don’t appear during the opening credits. They are locked, forcing you to unlock them.