Super Mario Party Jamboree has two secret characters to unlock, and despite what you might think, they’re pretty easy to find. You don’t have to grind or score any achievements to add them to your roster, and they’re fairly quick to find.
How to Unlock Ninji & Pauline in Super Mario Party Jamboree (HD)
These two characters are Pauline and Ninji, who make their Party debut and appear heavily in the game's commercials, but do not appear during the opening credits. They are locked, requiring you to unlock them.
While it may seem like both characters are hard to find, you can unlock them as soon as you start the game, without having to play Mario Party. Which begs the question: what was the point of all this?
Once you ascend in the hot air balloon and open the mode selection screen, you can freely look around the world to discover all the different modes and locations.