Buddies are characters from the movies that you can unlock after getting a certain amount of points in a stage, they are useful for collecting point coins and character coins for playable characters. You'll most likely get a Buddy after replaying the levels for a bit, but there's also a chance that you might get them in your first playthrough if you got enough points, a good stage for this is Daycare Dash, if you get 6000 coins, you'll both get a Gold Medal and Buzz as a Buddy, as it's pretty easy to get points in that stage. But it's best to leave this achievement for when you exactly start replaying the levels, due to the achievements being glitchy and not popping in your playthrough of a level due to the "1 achievement per level" limitation.